D17133CS datasheet(138/289 Pages) NEC

138 / 289 page

CHAPTER 13

PERIPHERAL HARDWARE

123

(2) Calculation method of count value

The time interval of the identity signal being emitted from the comparator is determined by the value that

is set in the modulo register. The formula for finding the value N of the modulo register from the time interval

T [sec] is shown below:

= (N+1)

× TCP

N=T

× fCP –1 or N=

–1 (N=1 to 255)

fCP : Count pulse's frequency [Hz]

TCP : Count pulse's frequency [sec] (1/fCP = resolution)

(3) Calculation example and program example when calculating count value by interval time

• Example of assuming 7 ms as interval time for timer (System clock: fX=8 MHz)

Assuming 7 ms as interval time, it is impossible to set 7 ms interval time from the resolution of the timer.

Therefore, count value should be calculated by selecting the source clock the resolution of which is

maximum (fX/256, resolution: 32

µs) to set the nearest interval time.

Example t=7 ms, resolution: 32

µs

–1

=– 1

= 217.75=218 (: DAH)

The value of modulo register the interval time of which becomes nearest to 7 ms is DAH, and the interval

time at that time becomes 7.008 ms.

fCP

N+1

TCP

(Resolution)

× 10–3

× 10–5

D17133CS Datasheet(PDF) 138 Page - NEC