NEC PD78214 - Manual (Page 214)

185
Chapter 7 Timer/Counter Units
7
Cautions 1. If a value less than the value of 8-bit timer 2 (TM2) is set in a compare register (CR20, CR21), a PWM signal with a 100% duty
factor is output. Rewrite the CR20 or CR21 compare register, if required, by using an interrupt generated by a coincidence
between TM2 and the compare register.
Fig. 7-89 Example of PWM Output Signal with a 100% Duty Factor
TO2
CR20
0H
n1 n2
n2
n2 n2
n3
n1
FFH
TM2
count value
When a value, n2 less than TM2 value, n3 is written to CR20
here, the duty factor is 100% during this period.
n1
FFH FFH FFH
Remark ALV2 = 0
2. If timer output is disabled (ENTOn = 0: n = 2, 3), the output level on the TOn (n = 2, 3) is the inverted value of the value set in
ALVn (n = 2, 3). Accordingly, note that if timer output is disabled when the PWM output function is selected, the active level
is output.
7.3.10 PPG Output
The PPG output function outputs a square wave that has a period determined by the CR21 compare register, and
has a pulse width determined by the CR20 compare register. PPG output is PWM output whose period is made
variable. This output signal can be output on the TO0 pin only.
Before this function can be used, the CLR21 bit of capture/compare control register 2 (CRC2) must be set to 1, the
CLR22 bit of the same register must be set to 0, and the CMD2 bit of timer control register 1 (TMC1) must be set
to 0.
The pulse period and pulse width are as follows:
• PPG period = ((value set in CR21 compare register) + 1) × x/f
CLK
; x = 16, 32, 64, 128, 256, 512
• PPG pulse width = ((value set in CR20 compare register)
× x + 2)/f
CLK
(value set in CR20)
× x/f
CLK
where, CR20
≤ CR21
• Duty factor = (PPG pulse width)/(PPG period) = ((value set in CR20)
× x + 2)/(((value set in CR21) + 1) × x) (value
set in CR20)/((value set in CR21) + 1)
Caution In PPG output, the actual pulse width is longer than a value obtained with the approximate expression by two clock pulses of
f
CLK
for the active level, and is shorter than such an approximate value by two clock pulses of f
CLK
for the inactive level. Take
this point into consideration when high-precision output is required, the PPG pulse period is short, or a high-speed count clock
is used.