21
8127B–AVR–08/09
ATtiny4/5/9/10
6.5 Register Description
6.5.1 CLKMSR – Clock Main Settings Register
• Bit 7:2 – Res: Reserved Bits
These bits are reserved and always read zero.
• Bit 1:0 – CLKMS[1:0]: Clock Main Select Bits
These bits select the main clock source of the system. The bits can be written at run-time to
switch the source of the main clock. The clock system ensures glitch free switching of the main
clock source.
The main clock alternatives are shown in Table 6-3.
To avoid unintentional switching of main clock source, a protected change sequence must be
followed to change the CLKMS bits, as follows:
1. Write the signature for change enable of protected I/O register to register CCP
2. Within four instruction cycles, write the CLKMS bits with the desired value
6.5.2 OSCCAL – Oscillator Calibration Register
.
• Bits 7:0 – CAL[7:0]: Oscillator Calibration Value
The oscillator calibration register is used to trim the calibrated internal oscillator and remove pro-
cess 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 16-2, “Calibration Accuracy of Internal RC Oscillator,” on page 118.
The application software can write this register to change the oscillator frequency. The oscillator
can be calibrated to frequencies as specified in Table 16-2, “Calibration Accuracy of Internal RC
Oscillator,” on page 118. Calibration outside the range given is not guaranteed.
The CAL[7:0] bits are used to tune the frequency of the oscillator. A setting of 0x00 gives the
lowest frequency, and a setting of 0xFF gives the highest frequency.
Bit 765432 1 0
0x37
– – – – – – CLKMS1 CLKMS0 CLKMSR
Read/WriteRRRRRRR/WR/W
Initial Value 0 0 0 0 0 0 0 0
Table 6-3. Selection of Main Clock
CLKM1 CLKM0 Main Clock Source
0 0 Calibrated Internal 8 MHzOscillator
0 1 Internal 128 kHz Oscillator (WDT Oscillator)
1 0 External clock
11Reserved
Bit 76543210
0x39 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 0 0 0 0 0 0 0 0
(375 strony)
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