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ATmega48PA/88PA/168PA [DATASHEET]
9223F–AVR–04/14
9.12
Register Description
9.12.1 OSCCAL – Oscillator Calibration Register
Bits 7:0 – CAL[7:0]: Oscillator Calibration Value
The oscillator calibration register is used to trim the calibrated internal RC oscillator to remove process variations from the
oscillator frequency. A pre-programmed calibration value is automatically written to this register during chip reset, giving the
factory calibrated frequency as specified in
Table 29-3 on page 271. The application software can write this register to
change the oscillator frequency. The oscillator can be calibrated to frequencies as specified in
Table 29-3 on page 271.
Calibration outside that range is not guaranteed.
Note that this oscillator is used to time EEPROM and flash write accesses, and these write times will be affected accordingly.
If the EEPROM or flash are written, do not calibrate to more than 8.8MHz. Otherwise, the EEPROM or flash write may fail.
The CAL7 bit determines the range of operation for the oscillator. Setting this bit to 0 gives the lowest frequency range,
setting this bit to 1 gives the highest frequency range. The two frequency ranges are overlapping, in other words a setting of
OSCCAL = 0x7F gives a higher frequency than OSCCAL = 0x80.
The CAL6...0 bits are used to tune the frequency within the selected range. A setting of 0x00 gives the lowest frequency in
that range, and a setting of 0x7F gives the highest frequency in the range.
9.12.2 CLKPR – Clock Prescale Register
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.
Bits 3:0 – CLKPS[3:0]: 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 synchronous peripherals is reduced when a division factor is used. The division factors are
Bit
7654
3210
CAL7
CAL6
CAL5
CAL4
CAL3
CAL2
CAL1
CAL0
OSCCAL
Read/Write
R/W
Initial Value
Device Specific Calibration Value
Bit
7
6543
2
1
0
CLKPCE
–
CLKPS3 CLKPS2 CLKPS1 CLKPS0
CLKPR
Read/Write
R/W
R
R/W
Initial Value
0
See Bit Description