User Manual
BCM1250/BCM1125/BCM1125H
10/21/02
B r o a d c o m C o r p o r a t i o n
Document
1250_1125-UM100CB-R
Section 3: System Overview
Page
31
The system_scratch register could be used as an alternative to using the configuration bit for the serial port.
The software would have to put a unique value into the register (or could set a bit that is reserved for this
purpose) before writing the system_reset bit.
C
LOCKS
The device is provided with a 100 MHz clock from which it generates the other internal clocks. The clock is
multiplied up with a PLL to provide the CPU clock, which is divided down to provide the internal bus clocks and
memory clock. A separate PLL is used to generate the HyperTransport clocks. An internal 100MHz clock is
generated either directly from the reference clock or by dividing the CPU clock by the PLL ratio to drive the
internal timers, the baud rate generator and for the timing parameters on the generic bus. The internal 100
MHz clock may be driven out on the IO_CLK100 pin. Using the reference source ensures that for all PLL ratios
the IO_CLK100 will have a duty cycle only a little worse than the reference clock and that the IO_CLK100 runs
at the same 100MHz frequency as the reference clock during cold reset and while the PLL locks. Using the
internally generated reference ensures that the IO_CLK100 tracks the internal clocks (it will run slowly during
cold reset even if the reference clock has not started), and if an integer PLL ratio is used (i.e. IO_AD[7] was
sampled low) ensures the duty cycle is no worse than 40:60 regardless of the reference clock. However, if a
nonintegral PLL ratio is used only the rising edge of the internally generated IO_CLK100 is valid, the time the
output spends high can vary from cycle to cycle and be as short as 10% of the cycle.
The PCI bus and network interfaces have their own bus clocks that must be externally generated to match the
attached components. The synchronous serial port clock either comes from the baud rate generator or from
an external source.
shows the supported clock ratios for the CPU and HyperTransport interface. Note that the speed grade
of the part determines the maximum frequency that is permitted. The ratio for the CPU is set statically using
the reset value on IO_AD[11:7] as described in the previous section. The ratio for the HyperTransport may be
set directly (by enabling an extension register in the HyperTransport configuration space), but is normally set
indirectly by encoding the value from the HyperTransport Link Frequency register.
Table 9: Core and HyperTransport Clock Settings
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)
00100
2x
200
100
200
400
00101
2.5x
250
125
250
500
00110
3x
300
150
300
600
00111
3.5x
350
175
350
700
01000
4x
400
200
400
800
01001
4.5x
450
225
450
900
01010
5x
500
250
500
1000
01011
5.5x
550
275
550
1100
01100
6x
600
300
600
1200
01101
6.5x
650
325
01110
7x
700
350
01111
7.5x
750
375
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