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Turbo PMAC User Manual
Turbo PMAC System Configuration and Auto Configuration
17
TURBO PMAC/PMAC2 SYSTEM CONFIGURATION AND
AUTO-CONFIGURATION
AUTO-CONFIGURATION
Turbo PMAC, and especially Turbo PMAC2, boards have extensive capabilities for automatically
identifying and self-configuring their systems. This is particularly important for UMAC Turbo systems,
with their wide variety of configurations. These capabilities provide ease of use and flexibility in getting
started with a particular configuration.
identifying and self-configuring their systems. This is particularly important for UMAC Turbo systems,
with their wide variety of configurations. These capabilities provide ease of use and flexibility in getting
started with a particular configuration.
CPU Clock Frequency
In any Turbo PMAC system, the clock frequency at which the CPU actually operates is set by software
variable I52, expressed as a multiplication factor from the fixed clock-crystal frequency for the system.
variable I52, expressed as a multiplication factor from the fixed clock-crystal frequency for the system.
The clock-crystal frequency on a Turbo PMAC is 19.66 MHz (sometimes called 20 MHz as a close
approximation). The CPU divides this in half, to 9.83 MHz, and then multiplies the resulting frequency
by an integer factor in a circuit called a phase-locked loop (PLL) to obtain its operating frequency. The
precise equation for the CPU operating frequency is:
approximation). The CPU divides this in half, to 9.83 MHz, and then multiplies the resulting frequency
by an integer factor in a circuit called a phase-locked loop (PLL) to obtain its operating frequency. The
precise equation for the CPU operating frequency is:
)
1
52
I
(
*
83
.
9
)
MHz
(
CPUfreq
+
=
In approximate terms, the equation is:
)
1
52
I
(
*
10
)
MHz
(
CPUfreq
+
=
Note that the frequency at which the CPU is operating is not necessarily the same as the maximum rated
frequency for the CPU. Depending on the CPU option ordered, the maximum rated frequency will be
different. The following table shows the maximum rated frequencies for each CPU option and the
maximum I52 values that can be used without exceeding these frequencies.
frequency for the CPU. Depending on the CPU option ordered, the maximum rated frequency will be
different. The following table shows the maximum rated frequencies for each CPU option and the
maximum I52 values that can be used without exceeding these frequencies.
CPU Option
CPU Max. Rated Freq.
Max. Rated I52 Value
Operating Freq. at
Max. Rated I52
5Cx
80 MHz
7
78.64 MHz
5Dx
100 MHz
9
98.30 MHz
5Ex
160 MHz
15
157.28 MHz
5Fx
240 MHz
23
235.92 MHz
The factory default value for I52 is 7, producing an 80 MHz CPU frequency, regardless of the CPU
option present. I52 is used only at power-up/reset time, so to change the CPU frequency, change the
active value of I52, store this new value to non-volatile flash memory with a SAVE command, and reset
the card (usually with a $$$ command). During the power-up/reset cycle, Turbo PMAC will read the
saved value of I52, set the PLL circuitry to generate the proper operating frequency, and make other
settings (e.g. baud rate dividers, memory and I/O wait states) appropriate for that frequency.
option present. I52 is used only at power-up/reset time, so to change the CPU frequency, change the
active value of I52, store this new value to non-volatile flash memory with a SAVE command, and reset
the card (usually with a $$$ command). During the power-up/reset cycle, Turbo PMAC will read the
saved value of I52, set the PLL circuitry to generate the proper operating frequency, and make other
settings (e.g. baud rate dividers, memory and I/O wait states) appropriate for that frequency.
Usually, I52 is set for the CPU to run at the maximum rated frequency. However, there are some cases in
which a lower frequency may be desired. If serial communications using either the main or auxiliary
serial port at 115,200 baud is desired, the CPU frequency must be an exact multiple of 30 MHz (29.49
MHz) to generate the baud rate accurately enough. So the Option 5Dx 100 MHz CPU might be operated
at 90 MHz to do this.
which a lower frequency may be desired. If serial communications using either the main or auxiliary
serial port at 115,200 baud is desired, the CPU frequency must be an exact multiple of 30 MHz (29.49
MHz) to generate the baud rate accurately enough. So the Option 5Dx 100 MHz CPU might be operated
at 90 MHz to do this.
Note:
It may be possible to operate a CPU at a frequency higher than its rated frequency,
particularly at moderate ambient temperatures. However, safe operation cannot be
guaranteed under these conditions, and any such operation is done entirely at the
user’s own risk.
particularly at moderate ambient temperatures. However, safe operation cannot be
guaranteed under these conditions, and any such operation is done entirely at the
user’s own risk.