44
7679H–CAN–08/08
AT90CAN32/64/128
AT90CAN32/64/128 share the Timer/Counter2 Oscillator Pins (TOSC1 and TOSC2) with PG4
and PG3. This means that both PG4 and PG3 can only be used when the Timer/Counter2 Oscil-
lator is not enable.
Applying an external clock source to TOSC1 can be done in asynchronous operation if EXTCLK
in the ASSR Register is written to logic one. See
“Asynchronous operation of the
for further description on selecting external clock as input instead
of a 32 kHz crystal. In this configuration, PG4 cannot be used but PG3 is available.
5.10
System Clock Prescaler
The AT90CAN32/64/128 system clock can be divided by setting the Clock Prescaler Register –
CLKPR. This feature can be used to decrease power consumption when the requirement for
processing power is low. This can be used with all clock source options, and it will affect the
clock frequency of the CPU and all synchronous peripherals. clk
I/O
, clk
ADC
, clk
CPU
, and clk
FLASH
are divided by a factor as shown in
.
5.10.1
Clock Prescaler Register – CLKPR
• Bit 7 – CLKPCE: Clock Prescaler Change Enable
The CLKPCE bit must be written to logic one to enable change of the CLKPS bits. The CLKPCE
bit is only updated when the other bits in CLKPR are simultaneously written to zero. CLKPCE is
cleared by hardware four cycles after it is written or when CLKPS bits are written. Rewriting the
CLKPCE bit within this time-out period does neither extend the time-out period, nor clear the
CLKPCE bit.
• Bit 6..0 – Reserved Bits
These bits are reserved for future use.
• Bits 3..0 – CLKPS3..0: Clock Prescaler Select Bits 3 - 0
These bits define the division factor between the selected clock source and the internal system
clock. These bits can be written run-time to vary the clock frequency to suit the application
requirements. As the divider divides the master clock input to the MCU, the speed of all synchro-
nous peripherals is reduced when a division factor is used. The division factors are given in
.
To avoid unintentional changes of clock frequency, a special write procedure must be followed
to change the CLKPS bits:
1.
Write the Clock Prescaler Change Enable (CLKPCE) bit to one and all other bits in
CLKPR to zero.
2.
Within four cycles, write the desired value to CLKPS while writing a zero to CLKPCE.
Interrupts must be disabled when changing prescaler setting to make sure the write procedure is
not interrupted.
The CKDIV8 Fuse determines the initial value of the CLKPS bits. If CKDIV8 is unprogrammed,
the CLKPS bits will be reset to “0000”. If CKDIV8 is programmed, CLKPS bits are reset to
Bit
7
6
5
4
3
2
1
0
CLKPCE
–
–
–
CLKPS3
CLKPS2
CLKPS1
CLKPS0
CLKPR
Read/Write
R/W
R
R
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
<-----
See Bit Description
----->
