49
ATmega103(L)
0945G–09/01
Timer/Counter1 Output
Compare Register – OCR1BH
and OCR1BL
The Output Compare registers are 16-bit read/write registers.
The Timer/Counter1 Output Compare registers contain the data to be continuously com-
pared with Timer/Counter1. Actions on compare matches are specified in the
Timer/Counter1 Control and Status registers. A compare match occurs only if
Timer/Counter1 counts to the OCR value. A software write that sets TCNT1 and OCR1A
or OCR1B to the same value does not generate a compare match.
A compare match will set the compare interrupt flag in the CPU clock cycle following the
compare event.
Since the Output Compare registers (OCR1A and OCR1B) are 16-bit registers, a tem-
porary register TEMP is used when OCR1A/B are written to ensure that both bytes are
updated simultaneously. When the CPU writes the high byte, OCR1AH or OCR1BH, the
data is temporarily stored in the TEMP register. When the CPU writes the low byte,
OCR1AL or OCR1BL, the TEMP register is simultaneously written to OCR1AH or
OCR1BH. Consequently, the high byte OCR1AH or OCR1BH must be written first for a
full 16-bit register write operation.
The TEMP register is also used when accessing TCNT1 and ICR1. If the main program
and interrupt routines perform access to registers using TEMP, interrupts must be dis-
abled during access from the main program.
Timer/Counter1 Input Capture
Register – ICR1H and ICR1L
The Input Capture Register is a 16-bit read-only register.
When the rising or falling edge (according to the input capture edge setting (ICES1)) of
the signal at the input capture pin – PD4(IC1) – is detected, the current value of the
Timer/Counter1 is transferred to the Input Capture Register (ICR1). At the same time,
the input capture flag (ICF1) is set (one).
Since the Input Capture Register (ICR1) is a 16-bit register, a temporary register TEMP
is used when ICR1 is read to ensure that both bytes are read simultaneously. When the
CPU reads the low byte ICR1L, the data is sent to the CPU and the data of the high byte
ICR1H is placed in the TEMP register. When the CPU reads the data in the high byte
ICR1H, the CPU receives the data in the TEMP register. Consequently, the low byte
ICR1L must be accessed first for a full 16-bit register read operation.
Bit
15
14
13
12
11
10
9
8
$29
MSB
OCR1BH
$28
LSB
OCR1BL
7
6
5
4
3
2
1
0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Bit
15
14
13
12
11
10
9
8
$27 ($37)
MSB
ICR1H
$26 ($36)
LSB
ICR1L
7
6
5
4
3
2
1
0
Read/Write
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
Initial Value
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
