ATTINY20_14 Datasheet, PDF (20/219 Pages) ATMEL Corporation – 8-bit AVR Microcontroller

Table 6-1. Start-up Times when Using the Internal Calibrated Oscillator
Reset
64 ms
Oscillator
6 cycles
Configuration
21 cycles
Total start-up time
64 ms + 6 oscillator cycles + 21 system clock cycles ⁽¹⁾⁽²⁾
Notes: 1. After powering up the device or after a reset the system clock is automatically set to calibrated internal 8
MHz oscillator, divided by 8
2. When the Brown-out Detection is enabled, the reset start-up time is 128 ms after powering up the device.
6.4.2
Starting from Power-Down Mode
When waking up from Power-Down sleep mode, the supply voltage is assumed to be at a sufficient level and only the
oscillator start-up time is counted to ensure the stable operation of the oscillator. The oscillator start-up time is counted
on the selected main clock, and the start-up time depends on the clock selected. See Table 6-2 for details.
Table 6-2. Start-up Time from Power-Down Sleep Mode
Oscillator start-up time
6 cycles
Total start-up time
6 oscillator cycles ⁽¹⁾⁽²⁾
Notes: 1. The start-up time is measured in main clock oscillator cycles.
2. When using software BOD disable, the wake-up time from sleep mode will be approximately 60 μs.
6.4.3
Starting from Idle / ADC Noise Reduction / Standby Mode
When waking up from Idle, ADC Noise Reduction or Standby Mode, the oscillator is already running and no oscillator
start-up time is introduced.
6.5 Register Description
6.5.1 CLKMSR – Clock Main Settings Register
Bit
0x37
Read/Write
Initial Value
7
6
5
4
3
2
1
0
–
–
–
–
–
–
CLKMS1
CLKMS0
CLKMSR
R
R
R
R
R
R
R/W
R/W
0
0
0
0
0
0
0
0
z Bits 7:2 – Res: Reserved Bits
These bits are reserved and will always read as zero.
z Bits 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.
ATtiny20 [DATASHEET] 20
Atmel-8235F-AVR-ATtiny20-Datasheet_09/2014