Functional Description
4.3.3
SDRAMC Register Programming
Several timing parameters are programmable when using SDRAM in a Intel® 440BX AGPset
system. The following table summarizes the programmable parameters.
Table 4-12. Programmable SDRAM Timing Parameters
Parameter
CAS# Latency
SDRAMC Bit
Values (DCLKs)
CL
SRCD
SRP
LCT
2,3
2,3
2,3
3,4
RAS# to CAS# Delay
RAS# Precharge
Leadoff CS# assertion
The 82443BX can support any combination of CAS# Latency, RAS# to CAS# Delay and RAS#
Precharge. Two additional bits are provided for controlling CS# assertion. The first is the Leadoff
Timing bits which effectively control when the command lines (SRAS#, SCAS# and WE#) are
considered valid on the interface and hence when CS# can be asserted for CPU read leadoff cycles.
In the fastest timing mode, CS# can be asserted in clock three. This enables a 7 clock page hit
performance with CAS# Latency two devices and one clock MD to HD delay. This field controls
when the first assertion of CS# occurs for read cycles initiated by the CPU. This assertion may be
for a read, row activate or precharge command. The MA lines along with the command lines
(SRAS#, SCAS# and WE#) are driven in clock two, however the clock to output delay timing is
slower than the other modes. Use of this mode may require a lightly loaded SDRAM interface.
4.3.4
DRAMT Register Programming
Various EDO timing parameters are programmable in the 82443BX. The ranges provide support
for the various loading configurations at 66 MHz. These are programmed via the DRAMT (DRAM
Timing) register. Only 60 ns EDO DRAMs are supported and at 66 MHz only. Thus, certain
parameters are fixed and are not programmable.
Table 4-13. EDO DRAM Timing Parameters
Parameter
60 ns EDO Spec (ns)
66 MHz CLKs
RAS# Precharge
40
60
20–45
10
15
0
3
RAS# Pulse Width
5
RAS# to CAS# Delay
CAS# Precharge
3
1
CAS# Pulse Width
1
WE# Setup to CAS# Falling
WE# Hold from CAS# Falling
MA Setup to RAS#/CAS#
MA Hold from RAS#/CAS#
MD Setup to CAS#
1
10
0
1
1 or 2
10
0
1
1
1
MD Hold from CAS#
10
82443BX Host Bridge Datasheet
4-23
ETC [ ETC ]
