Revision 1.02 – April 12, 2007
S5920 – PCI Product: Initialization
Data Book
the PCI address. For example, if the EXROM Base
Address is programmed with 100000h, and the PCI
performs a read to address 100040h, this will initiate a
read from address 40h of the nvRAM. Once addresses
40h, 41h, 42h and 43h have been read and stored in
the nvRAM interface, the S5920 is ready to provide
the data to the original PCI device requesting the data.
Once the original master comes back to read the data
(which it should, as it received a Retry to its initial
read), it will get a TRDY# along with the 4 bytes of
data that were read from the nvRAM. If the master
comes back to retry the read, but the nvRAM interface
is not finished with its accesses, the master will again
be greeted with a Retry. If a master attempts to read
from a different EXROM address, it will also be
greeted with a Retry. Only a read with the original
address (in our example, a read to address 100040h)
will allow the transaction to complete. As a result, if the
original master never comes back to Retry the read,
the EXROM interface will be hung. Only other EXROM
accesses will be hung, as the nvRAM interface will still
be operational via the PCI’s RCR and the Add-On’s
ARCR.
NON-VOLATILE MEMORY INTERFACE
The nv memory, can be accessed through the PCI
interface or the Add-On interface. Accesses to the nv
memory from the PCI interface are through the Reset
Control Register (RCR). Accesses to the nv memory
from the Add-On interface are through the Add-On
Reset Control Register (ARCR).
Some nv memories can contain Expansion ROM BIOS
code for use by the host CPU. During initialization, the
Expansion BIOS is located within system memory. The
starting location of the nv memory is stored in the
Expansion ROM Base Address Register in the S5920
PCI Configuration Registers. A PCI read from this
region results in the S5920 performing four consecu-
tive byte-wide access to the nv memory device, thus
assembling a complete DWORD. Writes to the nv
memory are not allowed through the expansion ROM
base address region. Any attempt to do so will result in
data being accepted by the S5920, but simply
discarded.
In the RCR and ARCR registers, bits D31:29 are com-
mand/status bits and bits D23:16 are address/data
bits. These operation registers occupy the same offset
(3Ch-3Fh) on their respective interfaces (Add-On or
PCI). The sequence used to access the nv memory is
the same in either case.
Accesses to the nvRAM via the PCI’s Reset Control
Register (RCR) are a bit more involved for the pro-
grammer. There are 12 bits of this register that perform
both reads and writes. Bits 23-16 to provide Address/
Data information, bits 31-29 are used to provide con-
trol information, and bit 28 indicates whether the
nvRAM access was successful or not. The control bits
31-29 are assigned as follows (where W/R indicates
the type of PCI access to the RCR):
nvRAM READ/WRITE DESCRIPTION
There are four different mechanisms to access the
external nvRAM:
1. During boot-up (RST# deasserted), the S5920
will automatically read out the nvRAM addresses
40h - 7Fh.
D31 D30 D29 W/R nvRAM Interface Function
0
1
1
1
1
0
1
X
0
0
1
1
X
X
X
0
1
0
1
X
X
W
W
W
W
W
R
Inactive
2. Via the PCI Configuration Expansion ROM Base
Address Register (EXROM). This is READ-ONLY.
Load low address byte
Load high address byte
Begin write
3. Via the PCI Reset Control Register (RCR). This is
READ/WRITE.
Begin read
Ready
4. Via the Add-On Reset Control Register (ARCR).
This is READ/WRITE.
R
Busy
The boot-up sequence is a built-in function, and is
affected by the contents of the nvRAM. The Expansion
ROM Base Address Register is used if expansion
BIOS is stored in the external nvRAM. This register
can be enabled for a 2K memory size, and is mapped
to access the contents of the nvRAM. When a read is
performed to an address in the range of the EXROM
base address, a read sequence is started to the
nvRAM. As this sequence is extremely slow, the PCI
will be greeted with a Retry. Mean-while, the nvRAM
interface circuitry will be performing four sequential
byte accesses to the nvRAM at the offset indicated by
These control bits are used along with the Address/
Data bits 23-16 to configure the type of nvRAM opera-
tion (read or write), the address being accessed, and a
place to store the write data or the data read from the
nvRAM. One can interface with this register in either
byte-wide or word-wide fashion. For a word-wide
access, the command (bits 31-29) and Address/Data
(bits 23-16) are written to the RCR with one PCI write.
For a byte-wide access, the command (bits 31-29) is
written first, followed by the Address/Data (bits 23-16).
This takes two PCI transfers.
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