Rainbow Electronics AT90S2313 - Manual (Page 36)

36
AT90S2313
0839I–AVR–06/02
During the time between the write and the latch operations, a read from OCR1A will
read the contents of the temporary location. This means that the most recently written
value always will read out of OCR1A.
When the OCR1 contains $0000 or TOP, the output OC1 is updated to low or high on
the next compare match according to the settings of COM1A1/COM1A0. This is shown
in Table 13.
Note: If the Compare Register contains the TOP value and the prescaler is not in use
(CS12..CS10 = 001), the PWM output will not produce any pulse at all, because the up-
counting and down-counting values are reached simultaneously. When the prescaler is in
use (CS12..CS10
≠ 001 or 000), the PWM output goes active when the counter reaches
the TOP value, but the down-counting compare match is not interpreted to be reached
before the next time the counter reaches the TOP value, making a one-period PWM
pulse.
In PWM mode, the Timer Overflow Flag1 (TOV1) is set when the counter advances from
$0000. Timer Overflow Interrupt1 operates exactly as in normal Timer/Counter mode
(i.e., it is executed when TOV1 is set, provided that Timer Overflow Interrupt1 and global
interrupts are enabled). This also applies to the Timer Output Compare1 Flag and
interrupt.
Table 13. PWM Outputs OCR = $0000 or TOP
COM1A1 COM1A0 OCR1A Output OC1
1 0 $0000 L
1 0 TOP H
1 1 $0000 H
1 1 TOP L