Datasheet

165
ATmega640/1280/1281/2560/2561
2549K–AVR–01/07
TCNT1H and TCNT1L –
Timer/Counter 1
TCNT3H and TCNT3L –
Timer/Counter 3
TCNT4H and TCNT4L –
Timer/Counter 4
TCNT5H and TCNT5L –
Timer/Counter 5
The two Timer/Counter I/O locations (TCNTnH and TCNTnL, combined TCN Tn) give
direct access, both for read and for write operations, to the Timer/Counter unit 16-bit
counter. To ensure that both the high and low bytes are read and written simultaneously
when the CPU accesses these registers, the access is performed using an 8-bit tempo-
rary High Byte Register (TEMP). This temporary register is shared by all the other 16-bit
registers. See “Accessing 16-bit Registers” on page 139.
Modifying the counter (TCNTn) while the counter is running introduces a risk of missing
a compare match between TCNTn and one of the OCRnx Registers.
Writing to the TCNTn Register blocks (removes) the compare match on the following
timer clock for all compare units.
Bit 76543210
(0x85) TCNT1[15:8] TCNT1H
(0x84) TCNT1[7:0] TCNT1L
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
Bit 76543210
(0x95) TCNT3[15:8] TCNT3H
(0x94) TCNT3[7:0] TCNT3L
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
Bit 76543210
(0xA5) TCNT4[15:8] TCNT4H
(0xA4) TCNT4[7:0] TCNT4L
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
Bit 76543210
(0x125) TCNT5[15:8] TCNT5H
(0x124) TCNT5[7:0] TCNT5L
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000