at90ls8535-8mi ATMEL Corporation, at90ls8535-8mi Datasheet - Page 40

• Manufacturer Part Number: at90ls8535-8mi
• Description: 8-bit Microcontroller With 8k Bytes In-system Programmable Flash
• Manufacturer: ATMEL Corporation

Timer/Counter1 Input Capture
Register – ICR1H AND ICR1L
Timer/Counter1 In PWM Mode
40
AT90S/LS8535
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.
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 (ICP) 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.
The TEMP register is also used when accessing TCNT1, OCR1A and OCR1B. If the
main program and interrupt routines perform access to registers using TEMP, interrupts
must be disabled during access from the main program.
When the PWM mode is selected, Timer/Counter1, the Output Compare Register1A
(OCR1A) and the Output Compare Register1B (OCR1B) form a dual 8-, 9- or 10-bit,
free-running, glitch-free and phase-correct PWM with outputs on the PD5(OC1A) and
PD4(OC1B) pins. Timer/Counter1 acts as an up/down counter, counting up from $0000
to TOP (see Table 14), where it turns and counts down again to zero before the cycle is
repeated. When the counter value matches the contents of the 10 least significant bits of
OCR1A or OCR1B, the PD5(OC1A)/PD4(OC1B) pins are set or cleared according to
the settings of the COM1A1/COM1A0 or COM1B1/COM1B0 bits in the Timer/Counter1
Control Register (TCCR1A). Refer to Table 15 for details.
Table 14. Timer TOP Values and PWM Frequency
Note that 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
Bit
$27 ($47)
$26 ($46)
Read/Write
Initial Value
PWM Resolution
10-bit
8-bit
9-bit
MSB
15
7
R
R
0
0
14
R
R
6
0
0
13
R
R
5
0
0
12
4
R
R
0
0
Timer TOP value
$03FF(1023)
$00FF (255)
$01FF (511)
11
R
R
3
0
0
10
R
R
2
0
0
R
R
9
1
0
0
LSB
Frequency
f
f
R
R
f
TCK1
TCK1
8
0
0
0
TCK1
/1022
/2046
/510
1041H–11/01
ICR1H
ICR1L