Serial presence detect

In computing, serial presence detect (SPD) is a standardized way to automatically access information about a memory module. Earlier 72-pin SIMMs included five pins that provided five bits of parallel presence detect (PPD) data, but the 168-pin DIMM standard changed to a serial presence detect to encode much more information.^[1]

When an ordinary modern computer is turned on, it starts by doing a power-on self-test (POST). Since about the mid-1990s, this process includes automatically configuring the hardware currently present. SPD is a memory hardware feature that makes it possible for the computer to know what memory is present, and what timings to use to access the memory.

Some computers adapt to hardware changes completely automatically. In most cases, there is a special optional procedure for accessing BIOS parameters, to view and potentially make changes in settings. It may be possible to control how the computer uses the memory SPD data—to choose settings, selectively modify memory timings, or possibly to completely over-ride the SPD data (see overclocking).

Stored information

For a memory module to support SPD, the JEDEC standards require that certain parameters be in the lower 128 bytes of an EEPROM located on the memory module. These bytes contain timing parameters, manufacturer, serial number and other useful information about the module. Devices utilizing the memory automatically determine key parameters of the module by reading this information. For example, the SPD data on an SDRAM module might provide information about the CAS latency so the system can set this correctly without user intervention.

The SPD EEPROM is accessed using SMBus, a variant of the I²C protocol. This reduces the number of communication pins on the module to just two: a clock signal and a data signal. The EEPROM shares ground pins with the RAM, has its own power pin, and has three additional pins (SA0–2) to identify the slot, which are used to assign the EEPROM a unique address in the range 0x50–0x57. Not only can the communication lines be shared among 8 memory modules, the same SMBus is commonly used on motherboards for system health monitoring tasks such as reading power supply voltages, CPU temperatures, and fan speeds.

(SPD EEPROMs also respond to I²C addresses 0x30–0x37 if they have not been write protected, and an extension uses addresses 0x18–0x1F to access an optional on-chip temperature sensor.^[2])

SDR SDRAM

Memory device on an SDRAM module, containing SPD data (red circled)

The first SPD specification was issued by JEDEC and tightened up by Intel as part of its PC100 memory specification.^[3] Most values specified are in binary coded decimal form. The most significant nibble can contain values from 10 to 15, and in some cases extends higher. In such cases, the encodings for 1, 2 and 3 are instead used to encode 16, 17 and 18. A most significant nibble of 0 is reserved to represent "undefined".

The SPD ROM defines up to three DRAM timings, for three CAS latencies specified by set bits in byte 18. First comes the highest CAS latency (fastest clock), then two lower CAS latencies with progressively lower clock speeds.

SPD contents for SDR SDRAM^[4]
Byte		Bit								Notes
(dec.)	(hex.)	7	6	5	4	3	2	1	0	Notes
0	0x00	Number of bytes present								Typically 128
1	0x01	log₂(size of SPD EEPROM)								Typically 8 (256 bytes)
2	0x02	Basic memory type (4: SPD SDRAM)
3	0x03	Bank 2 row address bits (0–15)				Bank 1 row address bits (1–15)				Bank 2 is 0 if same as bank 1
4	0x04	Bank 2 column address bits (0–15)				Bank 1 column address bits (1–15)				Bank 2 is 0 if same as bank 1
5	0x05	Number of RAM banks on module (1–255)								Commonly 1 or 2
6	0x06	Module data width low byte								Commonly 64, or 72 for ECC DIMMs
7	0x07	Module data width high byte								0, unless width ≥ 256 bits
8	0x08	Interface voltage level of this assembly (not the same as V_cc supply voltage) (0–4)								Decoded by table lookup
9	0x09	Nanoseconds (0–15)				Tenths of nanoseconds (0.0–0.9)				Clock cycle time at highest CAS latency
10	0x0a	Nanoseconds (0–15)				Tenths of nanoseconds (0.0–0.9)				SDRAM access time from clock (t_AC)
11	0x0b	DIMM configuration type (0–2): non-ECC, parity, ECC								Table lookup
12	0x0c	Self	Refresh period (0–5): 64, 256, 128, 32, 16, 8 kHz							Refresh requirements
13	0x0d	Bank 2 2×	Bank 1 primary SDRAM width (1–127, usually 8)							Width of bank 1 data SDRAM devices. Bank 2 may be same width, or 2× width if bit 7 is set.
14	0x0e	Bank 2 2×	Bank 1 ECC SDRAM width (0–127)							Width of bank 1 ECC/parity SDRAM devices. Bank 2 may be same width, or 2× width if bit 7 is set.
15	0x0f	Clock delay for random column reads								Typically 1
16	0x10	Page	—	—	—	8	4	2	1	Burst lengths supported (bitmap)
17	0x11	Banks per SDRAM device (1–255)								Typically 2 or 4
18	0x12	—	7	6	5	4	3	2	1	CAS latencies supported (bitmap)
19	0x13	—	6	5	4	3	2	1	0	CS latencies supported (bitmap)
20	0x14	—	6	5	4	3	2	1	0	WE latencies supported (bitmap)
21	0x15	—	Redundant	Diff. clock	Registered data	Buffered data	On-card PLL	Registered addr.	Buffered addr.	Memory module feature bitmap
22	0x16	—	—	Upper V_cc (supply voltage) tolerance	Lower V_cc (supply voltage) tolerance	Write/1 read burst	Precharge all	Auto-precharge	Early RAS precharge	Memory chip feature support bitmap
23	0x17	Nanoseconds (4–18)				Tenths of nanoseconds (0–9: 0.0–0.9)				Clock cycle time at medium CAS latency
24	0x18	Nanoseconds (4–18)				Tenths of nanoseconds (0–9: 0.0–0.9)				Data access time from clock (t_AC)
25	0x19	Nanoseconds (1–63)						0.25 ns (0–3: 0.00–0.75)		Clock cycle time at short CAS latency.
26	0x1a	Nanoseconds (1–63)						0.25 ns (0–3: 0.00–0.75)		Data access time from clock (t_AC)
27	0x1b	Nanoseconds (1–255)								Minimum row precharge time (t_RP)
28	0x1c	Nanoseconds (1–255)								Minimum row active–row active delay (t_RRD)
29	0x1d	Nanoseconds (1–255)								Minimum RAS to CAS delay (t_RCD)
30	0x1e	Nanoseconds (1–255)								Minimum active to precharge time (t_RAS)
31	0x1f	512 MiB	256 MiB	128 MiB	64 MiB	32 MiB	16 MiB	8 MiB	4 MiB	Module bank density (bitmap). Two bits set if different size banks.
32	0x20	Sign (1: −)	Nanoseconds (0–7)			Tenths of nanoseconds (0–9: 0.0–0.9)				Address/command setup time from clock
33	0x21	Sign (1: −)	Nanoseconds (0–7)			Tenths of nanoseconds (0–9: 0.0–0.9)				Address/command hold time after clock
34	0x22	Sign (1: −)	Nanoseconds (0–7)			Tenths of nanoseconds (0–9: 0.0–0.9)				Data input setup time from clock
35	0x23	Sign (1: −)	Nanoseconds (0–7)			Tenths of nanoseconds (0–9: 0.0–0.9)				Data input hold time after clock
36–61	0x24–0x3d	Reserved								For future standardization
62	0x3e	Major revision (0–9)				Minor revision (0–9)				SPD revision level; e.g., 1.2
63	0x3f	Checksum								Sum of bytes 0–62, not then negated
64–71	0x40–47	Manufacturer JEDEC id.								Stored little-endian, trailing zero-padded
72	0x48	Module manufacturing location								Vendor-specific code
73–90	0x49–0x5a	Module part number								ASCII, space-padded
91–92	0x5b–0x5c	Module revision code								Vendor-specific code
93	0x5d	Tens of years (0–9: 0–90)				Years (0–9)				Manufacturing date (YYWW)
94	0x5e	Tens of weeks (0–5: 0–50)				Weeks (0–9)				Manufacturing date (YYWW)
95–98	0x5f–0x62	Module serial number								Vendor-specific code
99–125	0x63–0x7f	Manufacturer-specific data								Could be enhanced performance profile
126	0x7e	0x66 [sic] for 66 MHz, 0x64 for 100 MHz								Intel frequency support
127	0x7f	CLK0	CLK1	CLK3	CLK3	90/100°C	CL3	CL2	Concurrent AP	Intel feature bitmap

DDR SDRAM

The DDR DIMM SPD format is an extension of the SDR SDRAM one. Mostly, parameter ranges are rescaled to accommodate higher speeds.

SPD contents for DDR SDRAM^[5]
Byte		Bit								Notes
(dec.)	(hex.)	7	6	5	4	3	2	1	0	Notes
0	0x00	Number of bytes written								Typically 128
1	0x01	log₂(size of SPD EEPROM)								Typically 8 (256 bytes)
2	0x02	Basic memory type (7 = DDR SDRAM)
3	0x03	Bank 2 row address bits (0–15)				Bank 1 row address bits (1–15)				Bank 2 is 0 if same as bank 1.
4	0x04	Bank 2 column address bits (0–15)				Bank 1 column address bits (1–15)				Bank 2 is 0 if same as bank 1.
5	0x05	Number of RAM banks on module (1–255)								Commonly 1 or 2
6	0x06	Module data width low byte								Commonly 64, or 72 for ECC DIMMs
7	0x07	Module data width high byte								0, unless width ≥ 256 bits
8	0x08	Interface voltage level of this assembly (not the same as V_cc supply voltage) (0–5)								Decoded by table lookup
9	0x09	Nanoseconds (0–15)				Tenths of nanoseconds (0.0–0.9)				Clock cycle time at highest CAS latency.
10	0x0a	Tenths of nanoseconds (0.0–0.9)				Hundredths of nanoseconds (0.00–0.09)				SDRAM access time from clock (t_AC)
11	0x0b	DIMM configuration type (0–2): non-ECC, parity, ECC								Table lookup
12	0x0c	Self	Refresh period (0–5): 64, 256, 128, 32, 16, 8 kHz							Refresh requirements
13	0x0d	Bank 2 2×	Bank 1 primary SDRAM width (1–127)							Width of bank 1 data SDRAM devices. Bank 2 may be same width, or 2× width if bit 7 is set.
14	0x0e	Bank 2 2×	Bank 1 ECC SDRAM width (0–127)							Width of bank 1 ECC/parity SDRAM devices. Bank 2 may be same width, or 2× width if bit 7 is set.
15	0x0f	Clock delay for random column reads								Typically 1
16	0x10	Page	—	—	—	8	4	2	1	Burst lengths supported (bitmap)
17	0x11	Banks per SDRAM device (1–255)								Typically 4
18	0x12	—	4	3.5	3	2.5	2	1.5	1	CAS latencies supported (bitmap)
19	0x13	—	6	5	4	3	2	1	0	CS latencies supported (bitmap)
20	0x14	—	6	5	4	3	2	1	0	WE latencies supported (bitmap)
21	0x15	—	x	Diff clock	FET switch external enable	FET switch on-board enable	On-card PLL	Registered	Buffered	Memory module feature bitmap
22	0x16	Fast AP	Concurrent auto precharge	Upper V_cc (supply voltage) tolerance	Lower V_cc (supply voltage) tolerance	—	—	—	Includes weak driver	Memory chip feature bitmap
23	0x17	Nanoseconds (0–15)				Tenths of nanoseconds (0.0–0.9)				Clock cycle time at medium CAS latency.
24	0x18	Tenths of nanoseconds (0.0–0.9)				Hundredths of nanoseconds (0.00–0.09)				Data access time from clock (t_AC)
25	0x19	Nanoseconds (0–15)				Tenths of nanoseconds (0.0–0.9)				Clock cycle time at short CAS latency.
26	0x1a	Tenths of nanoseconds (0.0–0.9)				Hundredths of nanoseconds (0.00–0.09)				Data access time from clock (t_AC)
27	0x1b	Nanoseconds (1–63)						0.25 ns (0–0.75)		Minimum row precharge time (t_RP)
28	0x1c	Nanoseconds (1–63)						0.25 ns (0–0.75)		Minimum row active–row active delay (t_RRD)
29	0x1d	Nanoseconds (1–63)						0.25 ns (0–0.75)		Minimum RAS to CAS delay (t_RCD)
30	0x1e	Nanoseconds (1–255)								Minimum active to precharge time (t_RAS)
31	0x1f	512 MiB	256 MiB	128 MiB	64 MiB	32 MiB	16 MiB/ 4 GiB	8 MiB/ 2 GiB	4 MiB/ 1 GiB	Module bank density (bitmap). Two bits set if different size banks.
32	0x20	Tenths of nanoseconds (0.0–0.9)				Hundredths of nanoseconds (0.00–0.09)				Address/command setup time from clock
33	0x21	Tenths of nanoseconds (0.0–0.9)				Hundredths of nanoseconds (0.00–0.09)				Address/command hold time after clock
34	0x22	Tenths of nanoseconds (0.0–0.9)				Hundredths of nanoseconds (0.00–0.09)				Data input setup time from clock
35	0x23	Tenths of nanoseconds (0.0–0.9)				Hundredths of nanoseconds (0.00–0.09)				Data input hold time after clock
36–40	0x24–0x28	Reserved								Superset information
41	0x29	Nanoseconds (1–255)								Minimum active to active/refresh time (t_RC)
42	0x2a	Nanoseconds (1–255)								Minimum refresh to active/refresh time (t_RFC)
43	0x2b	Nanoseconds (1–63, or 255: no maximum)						0.25 ns (0–0.75)		Maximum clock cycle time (t_CK max.)
44	0x2c	Hundredths of nanoseconds (0.01–2.55)								Maximum skew, DQS to any DQ. (t_DQSQ max.)
45	0x2d	Tenths of nanoseconds (0.0–1.2)				Hundredths of nanoseconds (0.00–0.09)				Read data hold skew factor (t_QHS)
46	0x2e	Reserved								For future standardization
47	0x2f	—						Height		Height of DIMM module, table lookup
48–61	0x30–0x3d	Reserved								For future standardization
62	0x3e	Major revision (0–9)				Minor revision (0–9)				SPD revision level, 0.0 or 1.0
63	0x3f	Checksum								Sum of bytes 0–62, not then negated
64–71	0x40–47	Manufacturer JEDEC id.								Stored little-endian, trailing zero-padded
72	0x48	Module manufacturing location								Vendor-specific code
73–90	0x49–0x5a	Module part number								ASCII, space-padded
91–92	0x5b–0x5c	Module revision code								Vendor-specific code
93	0x5d	Tens of years (0–90)				Years (0–9)				Manufacturing date (YYWW)
94	0x5e	Tens of weeks (0–50)				Weeks (0–9)				Manufacturing date (YYWW)
95–98	0x5f–0x62	Module serial number								Vendor-specific code
99–127	0x63–0x7f	Manufacturer-specific data								Could be enhanced performance profile

DDR2 SDRAM

The DDR2 SPD standard makes a number of changes, but is roughly similar to the above. One notable deletion is the confusing and little-used support for DIMMs with two ranks of different sizes.

For cycle time fields (bytes 9, 23, 25 and 49), which are encoded in BCD, some additional encodings are defined for the tenths digit to represent some common timings exactly:

DDR2 BCD extensions
Hex	Binary	Significance
A	1010	0.25 (¼)
B	1011	0.33 (⅓)
C	1100	0.66 (⅔)
D	1101	0.75 (¾)
E	1110	0.875 (⅞, nVidia XMP extension)
F	1111	Reserved

SPD contents for DDR2 SDRAM^[6]
Byte		Bit								Notes
Dec	Hex	7	6	5	4	3	2	1	0	Notes
0	0x00	Number of bytes written								Typically 128
1	0x01	log₂(size of SPD EEPROM)								Typically 8 (256 bytes)
2	0x02	Basic memory type (8 = DDR2 SDRAM)
3	0x03	Reserved				Row address bits (1–15)
4	0x04	Reserved				Column address bits (1–15)
5	0x05	Vertical height			Stack?	ConC?	Ranks−1 (1–8)			Commonly 0 or 1, meaning 1 or 2
6	0x06	Module data width								Commonly 64, or 72 for ECC DIMMs
7	0x07	Reserved
8	0x08	Interface voltage level of this assembly (not the same as V_cc supply voltage) (0–5)								Decoded by table lookup. Commonly 5 = SSTL 1.8 V
9	0x09	Nanoseconds (0–15)				Tenths of nanoseconds (0.0–0.9)				Clock cycle time at highest CAS latency.
10	0x0a	Tenths of nanoseconds (0.0–0.9)				Hundredths of nanoseconds (0.00–0.09)				SDRAM access time from clock (t_AC)
11	0x0b	DIMM configuration type (0–2): non-ECC, parity, ECC								Table lookup
12	0x0c	Self	Refresh period (0–5): 64, 256, 128, 32, 16, 8 kHz							Refresh requirements
13	0x0d	Primary SDRAM width (1–255)								Commonly 8 (module built from ×8 parts) or 16
14	0x0e	ECC SDRAM width (0–255)								Width of bank ECC/parity SDRAM devices. Commonly 0 or 8.
15	0x0f	Reserved
16	0x10	—	—	—	—	8	4	—	—	Burst lengths supported (bitmap)
17	0x11	Banks per SDRAM device (1–255)								Typically 4 or 8
18	0x12	7	6	5	4	3	2	—	—	CAS latencies supported (bitmap)
19	0x13	Reserved
20	0x14	—	—	Mini-UDIMM	Mini-RDIMM	Micro-DIMM	SO-DIMM	UDIMM	RDIMM	DIMM type of this assembly (bitmap)
21	0x15	—	Module is analysis probe	—	FET switch external enable	—	—	—	—	Memory module feature bitmap
22	0x16	—	—	—	—	—	—	—	Includes weak driver	Memory chip feature bitmap
23	0x17	Nanoseconds (0–15)				Tenths of nanoseconds (0.0–0.9)				Clock cycle time at medium CAS latency.
24	0x18	Tenths of nanoseconds (0.0–0.9)				Hundredths of nanoseconds (0.00–0.09)				Data access time from clock (t_AC)
25	0x19	Nanoseconds (0–15)				Tenths of nanoseconds (0.0–0.9)				Clock cycle time at short CAS latency.
26	0x1a	Tenths of nanoseconds (0.0–0.9)				Hundredths of nanoseconds (0.00–0.09)				Data access time from clock (t_AC)
27	0x1b	Nanoseconds (1–63)						1/4 ns (0–0.75)		Minimum row precharge time (t_RP)
28	0x1c	Nanoseconds (1–63)						1/4 ns (0–0.75)		Minimum row active–row active delay (t_RRD)
29	0x1d	Nanoseconds (1–63)						1/4 ns (0–0.75)		Minimum RAS to CAS delay (t_RCD)
30	0x1e	Nanoseconds (1–255)								Minimum active to precharge time (t_RAS)
31	0x1f	512 MiB	256 MiB	128 MiB	16 GiB	8 GiB	4 GiB	2 GiB	1 GiB	Size of each rank (bitmap).
32	0x20	Tenths of nanoseconds (0.0–1.2)				Hundredths of nanoseconds (0.00–0.09)				Address/command setup time from clock
33	0x21	Tenths of nanoseconds (0.0–1.2)				Hundredths of nanoseconds (0.00–0.09)				Address/command hold time after clock
34	0x22	Tenths of nanoseconds (0.0–0.9)				Hundredths of nanoseconds (0.00–0.09)				Data input setup time from strobe
35	0x23	Tenths of nanoseconds (0.0–0.9)				Hundredths of nanoseconds (0.00–0.09)				Data input hold time after strobe
36	0x24	Nanoseconds (1–63)						0.25 ns (0–0.75)		Minimum write recovery time (t_WR)
37	0x25	Nanoseconds (1–63)						0.25 ns (0–0.75)		Internal write to read command delay (t_WTR)
38	0x26	Nanoseconds (1–63)						0.25 ns (0–0.75)		Internal read to precharge command delay (t_RTP)
39	0x27	Reserved								Reserved for "memory analysis probe characteristics"
40	0x28	—	t_RC fractional ns (0–5): 0, 0.25, 0.33, 0.5, 0.66, 0.75			t_RFC fractional ns (0–5): 0, 0.25, 0.33, 0.5, 0.66, 0.75			t_RFC + 256 ns	Extension of bytes 41 and 42.
41	0x29	Nanoseconds (1–255)								Minimum active to active/refresh time (t_RC)
42	0x2a	Nanoseconds (1–255)								Minimum refresh to active/refresh time (t_RFC)
43	0x2b	Nanoseconds (0–15)				Tenths of nanoseconds (0.0–0.9)				Maximum clock cycle time (t_CK max)
44	0x2c	Hundredths of nanoseconds (0.01–2.55)								Maximum skew, DQS to any DQ. (t_DQSQ max)
45	0x2d	Hundredths of nanoseconds (0.01–2.55)								Read data hold skew factor (t_QHS)
46	0x2e	Microseconds (1–255)								PLL relock time
47–61	0x2f–0x3d	Reserved								For future standardization.
62	0x3e	Major revision (0–9)				Minor revision (0.0–0.9)				SPD revision level, usually 1.0
63	0x3f	Checksum								Sum of bytes 0–62, not negated
64–71	0x40–47	Manufacturer JEDEC ID								Stored little-endian, trailing zero-pad
72	0x48	Module manufacturing location								Vendor-specific code
73–90	0x49–0x5a	Module part number								ASCII, space-padded (limited to (,-,),A-Z,a-z,0-9,space)
91–92	0x5b–0x5c	Module revision code								Vendor-specific code
93	0x5d	Years since 2000 (0–255)								Manufacturing date (YYWW)
94	0x5e	Weeks (1–52)								Manufacturing date (YYWW)
95–98	0x5f–0x62	Module serial number								Vendor-specific code
99–127	0x63–0x7f	Manufacturer-specific data								Could be enhanced performance profile

DDR3 SDRAM

The DDR3 SDRAM standard significantly overhauls and simplifies the SPD contents layout. Instead of a number of BCD-encoded nanosecond fields, some "timebase" units are specified to high precision, and various timing parameters are encoded as multiples of that base unit.^[7] Further, the practice of specifying different time values depending on the CAS latency has been dropped; now there are just a single set of timing parameters.

Revision 1.1 lets some parameters be expressed as a "medium time base" value plus a (signed, −128 +127) "fine time base" correction. Generally, the medium time base is 1/8 ns (125 ps), and the fine time base is 1, 2.5 or 5 ps. For compatibility with earlier versions that lack the correction, the medium time base number is usually rounded up and the correction is negative. Values that work this way are:

DDR3 SPD two-part timing parameters
MTB byte	FTB byte	Value
12	34	t_CKmin, mimimum clock period
16	35	t_AAmin, mimimum CAS latency time
18	36	t_RCDmin, mimimum RAS# to CAS# delay
20	37	t_RPmin, mimimum row precharge delay
21,23	38	t_RCmin, mimimum active to active/precharge delay

SPD contents for DDR3 SDRAM^[8]
Byte		Bit								Notes
Dec	Hex	7	6	5	4	3	2	1	0	Notes
0	0x00	Exclude serial from CRC	SPD bytes total (undef/256)			SPD bytes used (undef/128/176/256)
1	0x01	SPD major revision				SPD minor revision				Typically 1.0 or 1.1
2	0x02	Basic memory type (11 = DDR3 SDRAM)								Type of RAM chips
3	0x03	Reserved				Module type				Type of module; e.g., 2 = Unbuffered DIMM, 3 = SO-DIMM, 11=LRDIMM
4	0x04	—	Bank address bits−3			log₂(bits per chip)−28				Zero means 8 banks, 256 Mibit.
5	0x05	—		Row address bits−12			Column address bits−9
6	0x06	Reserved					1.25 V	1.35 V	Not 1.5 V	Modules voltages supported. 1.5 V is default.
7	0x07	—		ranks−1			log₂(I/O bits/chip)−2			Module organization
8	0x08	—			ECC bits (001=8)		log₂(data bits)−3			0x03 for 64-bit, non-ECC DIMM.
9	0x09	Dividend, picoseconds (1–15)				Divisor, picoseconds (1–15)				Fine Time Base, dividend/divisor
10	0x0a	Dividend, nanoseconds (1–255)								Medium Time Base, dividend/divisor; commonly 1/8
11	0x0b	Divisor, nanoseconds (1–255)								Medium Time Base, dividend/divisor; commonly 1/8
12	0x0c	Minimum cycle time t_CKmin								In multiples of MTB
13	0x0d	Reserved
14	0x0e	11	10	9	8	7	6	5	4	CAS latencies supported (bitmap)
15	0x0f	—	18	17	16	15	14	13	12	CAS latencies supported (bitmap)
16	0x10	Minimum CAS latency time, t_AAmin								In multiples of MTB; e.g., 80/8 ns.
17	0x11	Minimum write recovery time, t_WRmin								In multiples of MTB; e.g., 120/8 ns.
18	0x12	Minimum RAS to CAS delay time, t_RCDmin								In multiples of MTB; e.g., 100/8 ns.
19	0x13	Minimum row to row active delay time, t_RRDmin								In multiples of MTB; e.g., 60/8 ns.
20	0x14	Minimum row precharge time, t_RPmin								In multiples of MTB; e.g., 100/8 ns.
21	0x15	t_RCmin, bits 11:8				t_RASmin, bits 11:8				Upper 4 bits of bytes 23 and 22
22	0x16	Minimum active to time, t_RASmin, bits 7:0								In multiples of MTB; e.g., 280/8 ns.
23	0x17	Minimum active to active/refresh, t_RCmin, bits 7:0								In multiples of MTB; e.g., 396/8 ns.
24	0x18	Minimum refresh recovery delay, t_RFCmin, bits 7:0								In multiples of MTB; e.g., 1280/8 ns.
25	0x19	Minimum refresh recovery delay, t_RFCmin, bits 15:8								In multiples of MTB; e.g., 1280/8 ns.
26	0x1a	Minimum internal write to read delay, t_WTRmin								In multiples of MTB; e.g., 60/8 ns.
27	0x1b	Minimum internal read to precharge delay, t_RTPmin								In multiples of MTB; e.g., 60/8 ns.
28	0x1c	Reserved				t_FAWmin, bits 11:8				In multiples of MTB; e.g., 240/8 ns.
29	0x1d	Minimum four activate window delay t_FAWmin, bits 7:0								In multiples of MTB; e.g., 240/8 ns.
30	0x1e	DLL-off	—	—	—	—	—	RZQ/7	RZQ/6	SDRAM optional features support bitmap
31	0x1f	PASR	—	—	—	ODTS	ASR	ETR 1×	ETR (95°C)	SDRAM thermal and refresh options
32	0x20	Present	Accuracy (TBD; currently 0 = undefined)							DIMM thermal sensor present?
33	0x21	Nonstd.	Die count			—	—	Signal load		Nonstandard SDRAM device type (e.g., stacked die)
34	0x22	t_CKmin correction (new for 1.1)								Signed multiple of FTB, added to byte 12
35	0x23	t_AAmin correction (new for 1.1)								Signed multiple of FTB, added to byte 16
36	0x24	t_RCDmin correction (new for 1.1)								Signed multiple of FTB, added to byte 18
37	0x25	t_RPmin correction (new for 1.1)								Signed multiple of FTB, added to byte 20
38	0x26	t_RCmin correction (new for 1.1)								Signed multiple of FTB, added to byte 23
39–59	0x27–0x3b	Reserved								For future standardization.
60	0x3c	—			Module height, mm (1–31, >45)					Module nominal height
61	0x3d	Back thickness, mm (1–16)				Front thickness, mm (1–16)				Module thickness, value = ceil(mm) − 1
62	0x3e	Design	Revision		JEDEC design number					JEDEC reference design used (11111=none)
63–116	0x3f–0x74	Module-specific section								Differs between registered/unbuffered
117	0x75	Module manufacturer ID, lsbyte								Assigned by JEP-106
118	0x76	Module manufacturer ID, msbyte								Assigned by JEP-106
119	0x77	Module manufacturing location								Vendor-specific code
120	0x78	Tens of years				Years				Manufacturing year (BCD)
121	0x79	Tens of weeks				Weeks				Manufacturing week (BCD)
122–125	0x7a–0x7d	Module serial number								Vendor-specific code
126–127	0x7e–0x7f	SPD CRC-16								Includes bytes 0–116 or 0–125; see byte 0 bit 7
128–145	0x80–0x91	Module part number								ASCII subset, space-padded
146–147	0x92–0x93	Module revision code								Vendor-defined
148–149	0x94–0x95	DRAM manufacturer ID								As distinct from module manufacturer
150–175	0x96–0xAF	Manufacturer-specific data

The memory capacity of a module can be computed from bytes 4, 7 and 8. The module width (byte 8) divided by the number of bits per chip (byte 7) gives the number of chips per rank. That can then be multiplied by the per-chip capacity (byte 4) and the number of ranks of chips on the module (usually 1 or 2, from byte 7).

Extensions

The JEDEC standard only specifies some of the SPD bytes. The truly critical data fits into the first 64 bytes,^[5]^[6]^[8]^[9]^[10] while some of the remainder is earmarked for manufacturer identification. However, a 256-byte EEPROM is generally provided. A number of uses have been made of the remaining space.

Enhanced Performance Profiles (EPP)

Memory generally comes with conservative timing recommendations in the SPD ROM, to ensure basic functionality on all systems. Enthusiasts often spend considerable time manually adjusting the memory timings for higher speed.

Enhanced Performance Profiles is an extension of SPD, developed by Nvidia and Corsair, which includes additional information for higher-performance operation of DDR2 SDRAM, including supply voltages and command timing information not included in the JEDEC SPD spec. The EPP information is stored in the same EEPROM, but in bytes 99-127, which are unused by standard DDR2 SPD.^[11]

EPP SPD ROM usage
Bytes	Size	Full profiles	Abbreviated profiles
99–103	5	EPP header
104–109	6	Profile FP1	Profile AP1
110–115	6	Profile FP1	Profile AP2
116–121	6	Profile FP2	Profile AP3
122–127	6	Profile FP2	Profile AP4

The parameters are particularly designed to fit the memory controller on the nForce 5, nForce 6 and nForce 7 chipsets. Nvidia encourages support for EPP in the BIOS for its high-end motherboard chipsets. This is intended to provide "one-click overclocking" to get better performance with minimal effort.

Nvidia's name for EPP memory that has been qualified for performance and stability is "SLI-ready memory".^[12] The term "SLI-ready-memory" has caused some confusion, as it has nothing to do with SLI video. One can use EPP/SLI memory with a single video card (even a non-Nvidia card), and one can run a multi-card SLI video setup without EPP/SLI memory.

An extended version, EPP 2.0, supports DDR3 memory as well.^[13]

Extreme Memory Profile (XMP)

A similar, Intel-developed JEDEC SPD extension for DDR3 SDRAM DIMMs. This uses bytes 176–255, which are unallocated by JEDEC, to encode higher-performance memory timings.^[14]

XMP SPD ROM usage^[15]
Bytes	Size	Use
176–184	10	XMP header
185–219	33	XMP profile 1 ("enthusiast" settings)
220–254	36	XMP profile 2 ("extreme" settings)

The header contains the following data. Most importantly, it contains a "medium timebase" value MTB, as a rational number of nanoseconds (common values are 1/8, 1/12 and 1/16 ns). Many other later timing values are expressed as an integer number of MTB units.

Also included in the header is the number of DIMMS per memory channel that the profile is designed to support; including more DIMMS may not work well.

XMP Header bytes^[15]
Byte	Bits	Use
176	7:0	XMP magic number byte 1 0x0C
177	7:0	XMP magic number byte 2 0x4A
178	0	Profile 1 enabled (if 0, disabled)
	1	Profile 2 enabled
	3:2	Profile 1 DIMMS per channel (1–4 encoded as 0–3)
	5:4	Profile 2 DIMMS per channel
	7:6	Reserved
179	3:0	XMP minor version number (x.0 or x.1)
179	7:4	XMP major version number (0.x or 1.x)
180	7:0	Medium timebase dividend for profile 1
181	7:0	Medium timebase divisor for profile 1 (MTB = dividend/divisor ns)
182	7:0	Medium timebase dividend for profile 2 (e.g. 8)
183	7:0	Medium timebase divisor for profile 2 (e.g. 1, giving MTB = 1/8 ns)
184	7:0	Reserved

XMP profile bytes^[15]
Byte 1	Byte 2	Bits	Use
185	220	0	Module Vdd voltage twentieths (0.00 or 0.05)
		4:1	Module Vdd voltage tenths (0.0–0.9)
		6:5	Module Vdd voltage units (0–2)
		7	Reserved
186	221	7:0	Minimum SDRAM clock period t_CKmin (MTB units)
187	222	7:0	Minimum CAS latency time t_AAmin (MTB units)
188	223	7:0	CAS latencies supported (bitmap, 4–11 encoded as bits 0–7)
189	224	6:0	CAS latencies supported (bitmap, 12–18 encoded as bits 0–6)
189	224	7	Reserved
190	225	7:0	Minimum CAS write latency time t_CWLmin (MTB units)
191	226	7:0	Minimum row precharge delay time t_RPmin (MTB units)
192	227	7:0	Minimum RAS to CAS delay time t_RCDmin (MTB units)
193	228	7:0	Minimum write recovery time t_WRmin (MTB units)
194	229	3:0	t_RASmin upper nibble (bits 11:8)
194	229	7:4	t_RCmin upper nibble (bits 11:8)
195	230	7:0	Minimum active to precharge delay time t_RASmin bits 7:0 (MTB units)
196	231	7:0	Minimum active to active/refresh delay time t_RCmin bits 7:0 (MTB units)
197	232	7:0	Maximum average refresh interval t_REFI lsbyte (MTB units)
198	233	7:0	Maximum average refresh interval t_REFI msbyte (MTB units)
199	234	7:0	Minimum refresh recovery delay time t_RFCmin lsbyte (MTB units)
200	235	7:0	Minimum refresh recovery delay time t_RFCmin msbyte (MTB units)
201	236	7:0	Minimum internal read to precharge command delay time t_RTPmin (MTB units)
202	237	7:0	Minimum row active to row active delay time t_RRDmin (MTB units)
203	238	3:0	t_FAWmin upper nibble (bits 11:8)
203	238	7:4	Reserved
204	239	7:0	Minimum four activate window delay time t_FAWmin bits 7:0 (MTB units)
205	240	7:0	Minimum internal write to read command delay time t_WTRmin (MTB units)
206	241	2:0	Write to read command turnaround time adjustment (0–7 clock cycles)
		3	Write to read command turnaround adjustment sign (0=pull-in, 1=push-out)
		6:4	Read to write command turnaround time adjustment (0–7 clock cycles)
		7	Read to write command turnaround adjustment sign (0=pull-in, 1=push-out)
207	242	2:0	Back-to-back command turnaround time adjustment (0–7 clock cycles)
		3	Back-to-back turnaround adjustment sign (0=pull-in, 1=push-out)
		7:4	Reserved
208	243	7:0	System CMD rate mode. 0=JTAG default, otherwise in peculiar units of MTB × t_CK/ns. E.g. if MTB is 1/8 ns, then this is in units of 1/8 clock cycle.
209	244	7:0	SDRAM auto self refresh performance. Standard version 1.1 says documentation is TBD.
210–218	245–253	7:0	Reserved
219	254	7:0	Reserved, vendor-specific personality code.

Vendor-specific memory

A common misuse is to write information to certain memory regions to bind vendor-specific memory modules to a specific system. Fujitsu Technology Solutions is known to do this. Adding different memory module to the system usually results in a refusal or other counter-measures (like pressing F1 on every boot).

02 0E 00 01-00 00 00 EF-02 03 19 4D-BC 47 C3 46 ...........M.G.F
53 43 00 04-EF 4F 8D 1F-00 01 70 00-01 03 C1 CF SC...O....p.....

This is the output of a 512 MB memory module from Micron Technologies, branded for Fujitsu-Siemens Computers, note the "FSC" string. The system BIOS rejects memory modules that don't have this information starting at offset 128h.

Reading and writing SPD information

Memory module manufacturers write the SPD information to the EEPROM on the module. Motherboard BIOSes read the SPD information to configure the memory controller. There exist several programs that are able to read and modify SPD information on most, but not all motherboard chipsets.

dmidecode program that can decode information about memory (and other things) and runs on Linux, FreeBSD, NetBSD, OpenBSD, BeOS, Cygwin and Solaris. dmidecode does not access SPD information directly; it reports the BIOS data about the memory.^[16] This information may be limited or incorrect.

On Linux systems, the user space program decode-dimms^[17] provided with i2c-tools ^[18] decodes and prints information on any memory with SPD information in the computer. It requires SMBus controller support in the kernel, the EEPROM kernel driver, and also that the SPD EEPROMs are connected to the SMBus. On older Linux distributions, decode-dimms.pl was available as part of lm_sensors.

OpenBSD has included a driver (spdmem(4)) since version 4.3 to provide information about memory modules. The driver was ported from NetBSD, where it is available since release 5.0.

Coreboot reads and uses SPD information to initialize all memory controllers in a computer with timing, size and other properties.

Windows systems use programs like HWiNFO32,^[19] CPU-Z and Speccy, which can read and display DRAM module information from SPD.

Chipset-independent reading and writing of SPD information is done by accessing the memory's EEPROM directly with eeprom programmer hardware and software.

A not so common use for old laptops is as generic SMBus readers, as the internal EEPROM on the module can be disabled once the BIOS has read it so the bus is essentially available for use. The method used is to pull low the A0,A1 lines so the internal memory shuts down, allowing the external device to access the SMBus. Once this is done, a custom Linux build or DOS application can then access the external device. A common use is recovering data from LCD panel memory chips to retrofit a generic panel into a proprietary laptop and also as a generic 24C0x/34C0x reader. This approach has been confirmed to work by A de Guerin using the Acer Extensa 5220/5630 series as these can both write and read DDR2 SPD chips using spdtool in Windows 7 x32 or x64 editions. As an interesting aside shorting A0 and A1 on an otherwise unresponsive laptop with two near identical memory modules during initial power-up sometimes allows it to boot so the data can be rewritten.

On older equipment

Some older equipment require the use of SIMMs with parallel presence detect (more commonly called simply presence detect or PD). Some of this equipment uses non-standard PD coding, IBM computers and Hewlett-Packard LaserJet and other printers in particular.

References

↑ Thomas P. Koenig; Nathan John (1997-02-03), "Serial Presence Detection poised for limelight", Electronic News 43 (2153)
↑ JEDEC Standard 21-C section 4.1.4 "Definition of the TSE2002av Serial Presence Detect (SPD) EEPROM with Temperature Sensor (TS) for Memory Module Applications"
↑ Application note INN-8668-APN3: SDRAM SPD Data Standards, memorytesters.com
↑ PC SDRAM Serial Presence Detect (SPD) Specification (PDF), 1.2A, December 1997, p. 28
1 2 JEDEC Standard 21-C section 4.1.2.4 "SPDs for DDR SDRAM"
1 2 JEDEC Standard 21-C section 4.1.2.10 "Specific SPDs for DDR2 SDRAM"
↑ Understanding DDR3 Serial Presence Detect (SPD) Table
1 2 JESD21-C Annex K: Serial Presence Detect for DDR3 SDRAM Modules, Release 4, SPD Revision 1.1
↑ JEDEC Standard 21-C section 4.1.2 "SERIAL PRESENCE DETECT STANDARD, General Standard"
↑ JEDEC Standard 21-C section 4.1.2.5 "Specific PDs for Synchronous DRAM (SDRAM)"
↑ DDR2 UDIMM Enhanced Performance Profiles Design Specification (PDF), Nvidia, 2006-05-12, retrieved 2009-05-05
↑ http://www.nvidia.com/docs/CP/45121/sli_memory.pdf
↑ Enhanced Performance Profiles 2.0 (pages 2–3)
↑ Intel Extreme Memory Profile (Intel XMP) DDR3 Technology
1 2 3 Intel^® Extreme Memory Profile (XMP) Specification, Rev 1.1 (PDF), October 2007, archived from the original (PDF) on 2012-03-06, retrieved 2010-05-25
↑ dmidecode: What's it good for?
↑ http://www.lm-sensors.org/browser/i2c-tools/trunk/eeprom/decode-dimms decode-dimms Perl program
↑ "I2CTools – lm-sensors". Lm-sensors.org. Retrieved 2014-08-14.
↑ HWiNFO32

External links

Serial Presence Detect Standard, General Standard
SPD Rev1.0 for DDR SDRAM
SPD Rev1.2 for DDR2 SDRAM
SPD Rev1.3 for DDR2 SDRAM
SPECIALITY DDR2-1066 SDRAM
Linux package i2c-tools
Instructions on how to use lm-sensors or i2c-tools to read the data
Memory Performance: 16GB DDR3-1333 to DDR3-2400 on Ivy Bridge IGP with G.Skill – explanation of various timing values

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