Chapter 3
DDR SDRAM Interface
177
24081D—February 2002
AMD-761™ System Controller Software/BIOS Design Guide
Preliminary Information
Periodic auto-calibration mode re-computes the Cal_Dly values
and transfers this value into the PDLs. All nine (or 18)
calibration mechanisms are enabled/disabled together for auto-
calibration. This mode is useful in adjusting the delay values
during operation. In effect, auto-calibration can adjust for
voltage and temperature (VT) drifts during operation. Note
that the AMD-761 system controller also allows re-calibration
to occur completely under software control when this (auto-
calibration) mode is disabled.
The auto-calibration period is configurable, and the possible
periods are 10000, 1000000, 10000000 clock cycles (at 100 MHz,
these periods are equal to 100
µ
s, 10 ms, and 100 ms, while at
133 MHz it is somewhat faster). The setting of the auto-
c a l i b r a t i o n p e r i o d s h o u l d b e b a s e d o n t h e a c t u a l
characteristics of the system.
Software can control when calibration is done (except for the
first computation at reset or an exit from self-refresh). It can
either configure the AMD-761 system controller for auto-
calibration (via the Auto_Cal_En bit), or it can initiate a single
recomputation (via the SW_ReCal bit). If software initiates a
single recomputation (via the SW_Recal bit), it should also poll
for this computation to be done.
Because auto-calibration registers are not initialized at reset, it
is the responsibility of the BIOS to initialize the SW_Cal_Dly.
The SW_Cal_Dly value that BIOS provides is based on a value
provided after AMD-761 silicon characterization. The hardware
computes the Cal_Dly value that is applied to the PDL based
on the SW_Cal_Dly programmed. The SW_Cal_Dly bits are
used by AMD-761 system controller to update the delay times
in both auto-calibration mode as well as software-initiated
calibrations. For example, if the delay required is 1.7 ns and
the system clock frequency is 133 MHz, the following is the
derivation of the SW_Cal_Dly value:
The half-period of system clock = 3.75 ns.
1.7 ns = 45.33% of the half-period.
The SW_Cal_Dly value is 0.4533 x 256 = 116 (rounded to
nearest integer) = 0x74.
The AMD-761 system controller allows software to optionally
write to the Act_Dly bits that control each PDL. The value
written to the Act_Dly bits is the number of buffer delays