
160
DDR SDRAM Interface
Chapter 3
AMD-761™ System Controller Software/BIOS Design Guide
24081D—February 2002
Preliminary Information
current. Byte 28 of the SPD defines the t
RRD
timing parameter.
Refer to Table 25 on page 160 for typical settings.
Write Recovery
The Write Recovery bits (Dev 0:F0:0x54, bit [25:24]) specify the
minimum amount of time that must occur from the last WRITE
command to a PRECHARGE command to the
same
internal
bank of the DDR device. This device timing parameter is not
specified in the SPD, but the recommended setting is 10b and
specifies two system clock cycles between a write command
and a precharge command to the
same
internal bank. Refer to
Table 25 on page 160 for typical settings.
Write to Read
The Write To Read bit (Dev 0:F0:0x54, bit [26]) specifies the
minimum amount of time that must occur between the last
WRITE command to a following READ command to the
same
internal bank of the DDR device. This device timing parameter
is not specified in the SPD, but the recommended setting is 1b
and specifies two system clock cycles. Refer to Table 25 on
page 160 for typical settings.
Table 25.
DDR Device Timing Values
Symbol
Name
SPD
Byte
Typical Value
Description
t
RCD
Minimum RAS to
CAS Delay
0x0x0x54[1:0]
29
50h
Has 2-bit fraction—see SPD
definitions. 50h = 20 ns.
2 cycles @100 MHz,
3 @ 133 MHz.
t
RAS
Minimum Active to
Precharge Time
0x0x0x54[6:4]
30
32h
Integer value. 50-ns require-
ment. 5 cycles @ 100 MHz,
7 @ 133 MHz.
t
RP
Minimum Row
Precharge Time
0x0x0x54[8:7]
27
50h
Has 2-bit fraction—see SPD
definitions. 50h = 20 ns.
2 cycles @100 MHz,
3 @ 133 MHz.
t
RC
Bank Cycle Time
0x0x0x54[11:9]
41
Typically defined as t
RAS
+ t
RP.
SPD entry available soon.
7 cycles @ 100 MHz,
10 @133 MHz.
t
RRD
Minimum Row Active
to Row Active Delay
0x0x0x54[23]
28
3Ch
Has 2-bit fraction—see SPD
definitions. 3Ch = 15 ns.
2 cycles @100 MHz and
133 MHz.
t
WR
Minimum Write to
Precharge Time
N/A
N/A
t
WTR
Minimum Write to
Read Time
N/A
N/A