Datasheet

48
ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]
2549Q–AVR–02/2014
10.13 Register Description
10.13.1 OSCCAL – Oscillator Calibration Register
• Bits 7:0 – CAL7:0: Oscillator Calibration Value
The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator to remove process varia-
tions 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 31-1 on page 359. The application
software can write this register to change the oscillator frequency. The oscillator can be calibrated to frequencies
as specified in Table 31-1 on page 359. 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 fre-
quency in that range, and a setting of 0x7F gives the highest frequency in the range.
10.13.2 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 – 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 synchronous peripherals is reduced when a division factor is used.
The division factors are given in Table 10-15 on page 49.
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 “0011”, giving a division factor of 8 at start
up. This feature should be used if the selected clock source has a higher frequency than the maximum frequency
of the device at the present operating conditions. Note that any value can be written to the CLKPS bits regardless
of the CKDIV8 Fuse setting. The Application software must ensure that a sufficient division factor is chosen if the
selected clock source has a higher frequency than the maximum frequency of the device at the present operating
conditions. The device is shipped with the CKDIV8 Fuse programmed.
Bit 76543210
(0x66) CAL7 CAL6 CAL5 CAL4 CAL3 CAL2 CAL1 CAL0 OSCCAL
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value Device Specific Calibration Value
Bit 7 6543210
(0x61)
CLKPCE – – – CLKPS3 CLKPS2 CLKPS1 CLKPS0 CLKPR
Read/Write R/W RRRR/W R/W R/W R/W
Initial Value 0 0 0 0 See Bit Description