Reference Manual

PMAC-PCI Hardware Reference
56 Software Setup
On a Flex CPU board configured for Option 5CF with 80 MHz maximum frequency, I46 should
be set to 7 to operate the CPU at its maximum rated frequency.
On a Flex CPU board configured for Option 5EF with 160 MHz maximum frequency, I46 should
be set to 15 to operate the CPU at its maximum rated frequency.
I46 is only used at power-up/reset, so to change the operational frequency, set a new value of I46,
issue a SAVE command to store this value in non-volatile flash memory, then issue a $$$
command to reset the controller.
To determine the frequency at which the CPU is actually operating, issue the TYPE command to
the PMAC. The PMAC will respond with five data items, the last of which is “CLK Xn”, where
“n” is the multiplication factor from the 20 MHz crystal frequency (not 10 MHz). “n” should be
equivalent to (I46+1)/2 if I46 is not requesting a frequency greater than the maximum rated for
that CPU board. “n” will be “2” for 40 MHz operation, “4” for 80 MHz operation, and “8” for
160 MHz operation.
• If the CPU’s operational frequency has been determined by (a non-zero setting of) I46, the serial
communications baud rate is determined at power-up/reset by variable I54 alone according to the
following table:
I54 Baud Rate I54 Baud Rate
0 600 8 9600
1 900 9 14,400
2 1200 10 19,200
3 1800 11 28,800
4 2400 12 38,400
5 3600 13 57,600
6 4800 14 76,800
7 7200 15 115,200
Note that these values can be different from those used on PMAC2 boards with jumper-set CPU
frequencies (see below).