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21555 Non-Transparent PCI-to-PCI Bridge User Manual 177
List of Registers
Table 109. ROM Setup Register
Byte Offsets: 0C9:0C8h
Bit Name R/W Description
1:0 Access Time R/W
Number of p_clk cycles that pr_cs_l asserts low (in default mode) or
pr_ale_l drives high (in multiple device mode) for a PROM or other external
device access. Possible values:
• 00: 8 times 33 MHz p_clk cycle time
00:16 times 66 MHz p_clk cycle time
• 01: 16 times 33 MHz p_clk cycle time
01: 32 times 66 MHz p_clk cycle time
• 10: 64 times 33 MHz p_clk cycle time
10:128 times 66 MHz p_clk cycle time
• 11: 256 times 33 MHz p_clk cycle time
11:512 times 66 MHz p_clk cycle time
A 33 MHz p_clk is specified by p_m66ena low, and a 66 MHz p_clk is
specified by p_m66ena high.
Reset value is 00b
7:2 Reserved R Reserved. Reads only as 0.
15:8 Strobe Mask R/W
Read and write strobe timing mask. This 8
-bit field defines the setup time,
duration, and hold time of pr_rd_l and pr_wr_l during the device select
assertion. A 1 asserts the strobe, a 0 deasserts it. The LSB is the first bit in
time, the MSB is the last bit. Each bit is one eighth of the access time, or for
the following access time values:
• 00: 1 each 33 MHz or 2 each 66 MHz p_clk cycles per bit
• 01: 2 each 33 MHz or 4 each 66 MHz p_clk cycles per bit
• 10: 8 each 33 MHz or 16 each 66 MHz p_clk cycles per bit
• 11: 32 each 33 MHz or 64 each 66 MHz p_clk cycles per bit
A 33 MHz p_clk is specified by p_m66ena low, and a 66 MHz p_clk is
specified by p_m66ena high.
Reset value is 01111110b
Table 110. ROM Data Register
Byte Offsets: 0CAh
Bit Name R/W Description
7:0 ROM_DATA R/W
• When the PROM Start bit is set, contains the read or write
data for bits [7:0] of the PROM.
• When the Serial ROM Start bit is set, contains the read or
write data for bits [7:0] of the serial ROM.