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PCI Functional Description
Configuration Registers
3-12
SYM53C825A/53C825AE Data Manual
Register 0Dh
Latency T imer
Read/Write
T he Latency T imer register specifies, in units of
PCI bus clocks, the value of the Latency T imer for
this PCI bus master. T he SYM53C825A supports
this timer. All eight bits are writable, allowing
latency values of 0-255 PCI clocks. Use the follow-
ing equation to calculate an optimum latency value
for the SYM53C825A:
Latency = 2 + (Burst Size * (typical wait states +1))
.
Values greater than optimum are also acceptable.
Register 0Eh
Header Type
Read Only
T his register identifies the layout of bytes 10h
through 3Fh in configuration space and also
whether or not the device contains multiple func-
tions. T he value of this register is 00h.
Register 10h
Base Address Zero (I/O)
Read/Write
T his 32-bit register has bit zero hardwired to one.
Bit 1 is reserved and must return a zero on all
reads, and the other bits are used to map the
device into I/O space.
Register 14h
Base Address One (Memory)
Read/Write
T his register has bit 0 hardwired to zero. For
detailed information on the operation of this regis-
ter, refer to the PCI Specification.
Register 18h
RAM Base Address
Read/Write
T his register holds the memory base address of the
4 K B internal RAM. T he user can read this regis-
ter through the SCRAT CHB register in the oper-
ating register set when bit 3 of the CT EST 2
register is set.
Register 2Ch
Subsystem Vendor ID (SSVID)
Read Only
T his register supports subsystem identification,
which has a default value of 1000h in the
SYM53C825AE (see Mad Bus Programming in
Chapter 4). To write to this register, connect a
4.7 K
resistor between the MAD(6) pin and V
ss
and leave the MAD(4) pin unconnected. T he
MAD(6) and MAD(4) pins have internal pull-up
resistors and are sensed shortly after the deasser-
tion of chip reset. In revisions before Rev. G of the
SYM53C825A, the MAD(6) and MAD(4) pins
do not support the SSID and SSVID configura-
tions, and only values of 0000h can be found in the
Subsystem Data register.
Register 2Eh
Subsystem ID (SSID)
Read Only
T his register supports subsystem identification,
which has a default value of 1000h in the
SYM53C825AE (see Mad Bus Programming in
Chapter 4). To write to this register, connect a
4.7 K
resistor between the MAD(6) pin and V
ss
and leave the MAD(4) pin unconnected. T he
MAD(6) and MAD(4) pins have internal pull-up
resistors and are sensed shortly after the deasser-
tion of chip reset. In revisions before Rev. G of the
SYM53C825A, the MAD(6) and MAD(4) pins
do not support the SSID and SSVID configura-
tions, and only values of 0000h can be found in the
Subsystem Data register.