Chapter 3
DDR SDRAM Interface
159
24081D—February 2002
AMD-761™ System Controller Software/BIOS Design Guide
Preliminary Information
t
RCD
The RAS to CAS delay bits (Dev 0:F0:0x54, bits [1:0]) specify
the minimum amount of time required between the opening of
a page within the DDR device (via an ACTIVATE command)
and the issuance of a READ or WRITE command to that same
DDR device’s internal bank. This timing parameter is device-
specific. Byte 29 of the SPD defines the t
RCD
timing parameter.
Refer to Table 25 on page 160 for typical settings.
t
RAS
The Row Active bits (Dev 0:F0:0x54, bits [6:4]) specify the
minimum amount of time that a page within the DDR device
(via an ACTIVATE command) can remain opened within the
same internal bank of the DDR device. This timing parameter
is device-specific. Byte 30 of the SPD defines the t
RAS
timing
parameter. Refer to Table 25 on page 160 for typical settings.
t
RP
The Row Precharge time bits (Dev 0:F0:0x54, bits [8:7]) specify
the minimum amount of time that the DDR device requires to
precharge a row and is specified as the time between the
PRECHARGE command and an ACTIVATE command within
the
same
internal bank of the DDR device. This timing
parameter is DDR device-specific. Byte 27 of the SPD defines
the t
RP
timing parameter. Refer to Table 25 on page 160 for
typical settings.
t
RC
The Row Cycle time bits (Dev 0:F0:0x54, bits [11:9]) specify the
minimum amount of time that the DDR device requires
between ACTIVATE commands within the
same
internal bank
of the DDR device. This timing parameter is DDR device-
specific. In short, this requirement specifies the minimum
amount of time that the
same
internal bank can recycle row
accesses. Byte 41 of the SPD defines the t
RC
timing parameter.
(Note that t
RC
is new to the SPD and voted in at the September
2000 JEDEC meeting.) Historically, t
RC
was defined as t
RAS
+
t
RP
, but this algorithm is not recommended when the SPD
information is available. Refer to Table 25 on page 160 for
typical settings.
t
RRD
The Bank to Bank ACTIVATE time bit (Dev 0:F0:0x54, bit [23])
specifies the minimum amount of time that the
same
DDR
device can receive back-to-back ACTIVATE commands, even to
different
internal banks. This timing parameter is DDR device-
specific. Device manufacturers specify the t
RRD
parameter to
limit current surges within the device, based on row ACTIVATE
activity, because row activates require a large amount of