Toshiba TLCS-90 Series Data Book page 273

TOSHIBA TMP90C840
<D
Prescaler
An 9-bit prescaler is provided to further devide the clock frequency
already divided to a 1/4 of the frequency of the source clock
(fC).
It generates a input clock pulse for the 8-bit timers, 16-bit
timer/event counter, the baud-rate generator, etc.
For the 8-bit timers, three types of clock are generated (oT I, oT 16
and oT256).
The prescaler can be run or stopped by using the 5th bit TRUN5 (PRRUN)
of
th~
timer control register (TRUN). Set t ing TRUNS to "I" makes the
prescaler count, and setting it to "0" clears the prescaler to stop.
By resetting, TRUNS is initialized to "0", making the prescaler cleare
and stop.
Oscillator
circuit
B=
¢l
1/2
¢2
eLK
fc/4
¢Tl
¢T4
fc/4
¢Tl ¢T4 ¢T16 ¢T64 ¢T256
a
1 2 3 4 5 6 7 8
9 Bit Prescaler
run/stop & clear
TRUN5
Fig. 3.6 (2) Prescaler
MPU90-75