Rainbow Electronics ATmega128RFA1 - Manual (Page 307)

307
8266A-MCU Wireless-12/09
ATmega128RFA1
20 Output Compare Modulator (OCM1C0A)
20.1 Overview
The Output Compare Modulator (OCM) allows generation of waveforms modulated with
a carrier frequency. The modulator uses the outputs from the Output Compare Unit C of
the 16-bit Timer/Counter1 and the Output Compare Unit of the 8-bit Timer/Counter0.
For more details about these Timer/Counters see
"Timer/Counter 0, 1, 3, 4, and 5
Prescaler" on page 304 and "8-bit Timer/Counter2 with PWM and Asynchronous
Operation" on page 309 .
Figure 20-1. Output Compare Modulator, Block Diagram
OC1C
Pin
OC1C /
OC0A / PB7
Timer/Counter 1
Timer/Counter 0 OC0A
When the modulator is enabled, the two output compare channels are modulated
together as shown in the block diagram (
Figure 20-1 above ).
20.2 Description
The Output Compare unit 1C and Output Compare unit 2 share the PB7 port pin for
output. The outputs of the Output Compare units (OC1C and OC0A) override the
normal PORTB7 Register when one of them is enabled (i.e., when COMnx1:0 is not
equal to zero). When both OC1C and OC0A are enabled at the same time, the
modulator is automatically enabled.
The functional equivalent schematic of the modulator is shown on in the following
figure. The schematic includes part of the Timer/Counter units and the port B bit 7
output driver circuit.
Figure 20-2. Output Compare Modulator, Schematic
PORTB7 DDRB7
D Q D Q
Pin
COMA01
COMA00
DATABUS
OC1C /
OC0A/ PB7
COM1C1
COM1C0
Modulator
1
0
OC1C
D Q
OC0A
D Q
( From Waveform Generator )
( From Waveform Generator )
0
1
Vcc