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
32
Section 3: System Overview
Document
1250_1125-UM100CB-R
The clocks for the I/O bridges that connect the peripherals to the ZBbus (see
) are
synchronously divided down from the CPU clock. The divide ratio needs to be set as part of the reset time
configuration (see
) based on the CPU frequency. The I/O Bridge clock does not
affect the speed of the peripheral interfaces directly, but it does affect the bandwidth (and latency) between the
peripheral and the ZBbus. Increasing the bridge clock speed will increase the bandwidth, but will use more
power. Conversely the speed can be decreased to save power if the full bandwidth is not needed. I/O Bridge
0 is designed for operation with the clock about 200 MHz, it should be set in the range 166-266 MHz (check
the data sheet for the maximum frequency for a given speed grade). I/O Bridge 1 is designed for operation with
the clock about 266 MHz, it should be set in the range 233-333 MHz (check the data sheet for the maximum
frequency for a given speed grade). The maximum frequency for the bridge clocks is given in the Data Sheet
as Fbr0 and Fbr1 in the Clock, Reset and Test timing parameters.
10000
8x
800
400
10001
8.5x
850
425
10010
9x
900
450
10011
9.5x
950
475
10100
10x
1000
500
10101
10.5x
1050
525
10110
11x
1100
550
Table 9: Core and HyperTransport Clock Settings
(Cont.)
Code
Ratio
Main PLL (Code from Reset
Time IO_AD[11:7])
HyperTransport PLL (Code from HyperTransport
Frequency Register)
CPU Clock
(MHz)
ZBbus Clock
(MHz)
HyperTransport Clock
(MHz)
HyperTransport Data Rate
(Mbps/pair)