Rainbow Electronics ATmega8515L - Manual (Page 103)

103
ATmega8515(L)
2512A–AVR–04/02
measuring frequency only, the clearing of the ICF1 flag is not required (if an interrupt
handler is used).
Output Compare Units The 16-bit comparator continuously compares TCNT1 with the Output Compare Regis-
ter (OCR1x). If TCNT equals OCR1x the comparator signals a match. A match will set
the
Output Compare Flag (OCF1x) at the next timer clock cycle. If enabled (OCIE1x =
1), the output compare flag generates an output compare interrupt. The OCF1x flag is
automatically cleared when the interrupt is executed. Alternatively the OCF1x flag can
be cleared by software by writing a logical one to its I/O bit location. The waveform gen-
erator uses the match signal to generate an output according to operating mode set by
the
Waveform Generation mode (WGM13:0) bits and Compare O utput mode
(COM1x1:0) bits. The TOP and BOTTOM signals are used by the waveform generator
for handling the special cases of the extreme values in some modes of operation. See
“Modes of Operation” on page 106.
A special feature of output compare unit A allows it to define the Timer/Counter TOP
value (i.e., counter resolution). In addition to the counter resolution, the TOP value
defines the period time for waveforms generated by the waveform generator.
Figure 49 shows a block diagram of the output compare unit. The small “n” in the regis-
ter and bit names indicates the device number (n = 1 for Timer/Counter1), and the “x”
indicates output compare unit (A/B). The elements of the block diagram that are not
directly a part of the output compare unit are gray shaded.
Figure 49. Output Compare Unit, Block Diagram
The OCR1x register is double buffered when using any of the twelve Pulse Width Modu-
lation (PWM) modes. For the normal and Clear Timer on Compare (CTC) modes of
operation, the double buffering is disabled. The double buffering synchronizes the
update of the OCR1x Compare Register to either TOP or BOTTOM of the counting
OCFnx (Int.Req.)
= (16-bit Comparator )
OCRnx Buffer (16-bit Register)
OCRnxH Buf. (8-bit)
OCnx
TEMP (8-bit)
DATA BUS
(8-bit)
OCRnxL Buf. (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit) TCNTnL (8-bit)
COMnx1:0 WGMn3:0
OCRnx (16-bit Register)
OCRnxH (8-bit) OCRnxL (8-bit)
Waveform Generator
TOP
BOTTOM