Intel 386 - Manual (Page 188)

8-7
CLOCK AND POWER MANAGEMENT UNIT
8.2 CONTROLLING THE PSCLK FREQUENCY
The PSCLK signal can provide a 50% duty cycle prescaled clock to the timer/counter and SSIO
units. This feature is useful for providing various frequencies, including a 1.19318 MHz output
for a PC-compatible system timer, or speaker tone generator. Determine the required prescale val-
ue using the following formula, then write this value to the CLKPRS register (Figure 8-4).
Figure 8-4. Clock Prescale Register (CLKPRS)
To change the frequency of PSCLK, write a new value to the CLKPRS register. The new frequen-
cy takes effect at the first high-to-low transition of PSCLK after CLKPRS has been changed.
Clock Prescale Register
CLKPRS
(read/write) Expanded Addr:
ISA Addr:
Reset State: F804H
—
0000H
15 8
— — — — — — — PS8
7 0
PS7 PS6 PS5 PS4 PS3 PS2 PS1 PS0
Bit
Number
Bit
Mnemonic
Function
15–9 — Reserved. These bits are undefined; for compatibility with future devices,
do not modify these bits.
8–0 PS8:0 Prescale Value:
These bits determine the divisor that is used to generate PSCLK. Legal
values are from 0000H (divide by 2) to 01FFH (divide by 513).
divisor = PS8:0 + 2
Prescale value
internal clock frequency (CLK2/2)
desired PSCLK frequency
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