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Serial ROM Interface
This chapter presents the theory of operation information about the 21555 Serial ROM (SROM) interface. See
Chapter 16
for specific information about the SROM registers.
The serial ROM interface is used to preload data into the 21555 configuration registers with vendor-specific values.
The format for the serial ROM data is given in
(VPD) interface as described in
The SROM interface works with the Microchip 93LC66A* or compatible SROM, which is a byte-organized
Microwire SROM with 4 Kb (512 bytes) of storage. The clock input to the SROM is the primary clock input, p_clk,
operating at a maximum clock frequency of 33 MHz divided by 34. When p_clk is operating above 33 MHz, it is
divided by 68 to generate the SROM clock input. The duty cycle is approximately 50%.
9.1
SROM Interface Signals
The SROM interface consists of four signals, as shown in
other signals are multiplexed with the PROM (PROM) interface signals (refer to
attached directly to the SROM pins without additional external logic.
Table 22. SROM Interface Signals
Name
sr_cs
sr_ck
sr_di
sr_do
9.2
SROMSROM Preload Operation
The SROM interface is used to preload the 21555 configuration registers whenever the 21555 configuration
registers are reset, either through assertion of p_rst_l or s_rst_l, by setting the Chip Reset bit in the Chip Control
Register or after a power management transition from D3
Once reset is complete, the 21555 automatically starts a serial read from the ROM by detecting that p_rst_l and
s_rst_l are deasserted and the Chip Reset bit reset to 0 (zero). All of the 21555 initialization data is loaded with a
single read operation by keeping the chip select asserted and toggling the clock. The 21555 returns a target retry to
all configuration accesses until the preload operation is complete. The preload operation takes approximately 550
SROM clock cycles.
When the SROM is not present, the sr_do (pin pr_ad[2]) should be pulled up through an external resistor. When the
SROM is present but register preload is not desired, bits [7:6] of the first byte (the first two bits read) can be any
value except the preload enable sequence 10b. When the 21555 does not detect the preload enable sequence when
reading the first byte, it stops the preload operation. In this case, all configuration registers preloaded with the
SROM remain at their reset value and should be initialized by the local processor before host access.
21555 Non-Transparent PCI-to-PCI Bridge User Manual
Section
Chapter
15.
Type
Description
O
Serial ROM Chip Select
O
Serial ROM Clock
O
Serial ROM Data In
I
Serial ROM Data Out
9.3. The SROM can be support the Vital Product Data
Table
22. The chip select, sr_cs, has a dedicated pin. The
21555 Pin
sr_cs
pr_ad[0]
pr_ad[1]
pr_ad[2]
to D0.
hot
Table
21). The SROM may be
9
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