BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
372
Section 11: Generic/Boot Bus
Document
1250_1125-UM100CB-R
least 1 (or more if the ready deasserts longer after the strobe). The rdy_smpl value may be used to widen the
strobe if required.
If the burst mode is selected with a non-multiplexed bus the address will not increment through the burst, it will
remain at the start address of the burst for the entire access.
E
ARLY
C
HIP
S
ELECT
The chip select normally asserts ale_to_cs cycles after the IO_ALE deasserts. Some peripherals will only take
note of the IO_ALE when their chip select is asserted. The early_cs bit in the
io_ext_time_cfg0
register can
be set to support them. It causes the external chip select signal to be asserted at the same time as IO_ALE
and remain asserted through the ale_to_cs delay and the normal chip select assertion time. Internally the
normal chip select timing is used for control of the cycle.
B
OOT
ROM S
UPPORT
After reset, chip select region 0 is configured to map 4MB starting at the physical address location of the MIPS
processor reset exception routine,
00_1FC0_0000
. The timing parameters for the region are set for a slow
(40 ns) address latch connected to a slow ROM with 240 ns access time from chip select and output enable.
The other configuration parameters are set based on the boot_mode configuration resistors as described in
The SMBus boot modes still use the generic bus module. The SMBus interface 0 protocol engine is placed
under the control of the generic bus controller. Rather than run a generic bus cycle to fetch 32 bits of data, a
request is made to the SMBus controller. The SMBus interface can be returned to normal software control by
clearing the boot_mode[1] bit (bit[18] in the
system_cfg
register), once this is done the behavior of chip select
region 0 becomes UNDEFINED until the
io_ext_cfg_0
,
io_ext_time_cfg0_0
and
io_ext_time_cfg1_0
registers have been written.
Table 250: Generic Bus Configuration for Each Boot Mode
Boot Mode
Generic Configuration
Comments
00
Width = 32 bit / Fixed timing / Multiplexed A/D /
No Parity
This gives the highest performance ROM connection.
01
Width = 8 bit / Fixed timing / NonMultiplexed A/D
/ No Parity
If address bits [27:24] and the byte enables (on bits
[31:28]) are not needed, this will work with an 8 bit ROM
connected for a multiplexed A/D bus.
10
Width = 32 bit / Acknowledgement mode /
Timeout disabled / Access small EEPROM using
SMBus
These two boot modes use the SMBus boot path
described in section
Section: “Booting Using an SMBus
.
11
Width = 32 bit / Acknowledgement mode /
Timeout disabled/ Access large EEPROM using
SMBus