Clock Sources

The device has the following clock source options, selectable by Flash Fuse Bits as shown below. The clock from the selected source is input to the AVR clock generator, and routed to the appropriate modules.

Table 1. Device Clocking Options Select(1)
Device Clocking Option CKSEL3:0
External Crystal/Ceramic Resonator 1111 - 1010
External Low-frequency Crystal 1001
External RC Oscillator 1000 - 0101
Calibrated Internal RC Oscillator 0100 - 0001
External Clock 0000
Note: 1. For all fuses “1” means unprogrammed while “0” means programmed.

The various choices for each clocking option is given in the following sections. When the CPU wakes up from Power-down or Power-save, the selected clock source is used to time the start-up, ensuring stable Oscillator operation before instruction execution starts. When the CPU starts from reset, there is as an additional delay allowing the power to reach a stable level before commencing normal operation. The Watchdog Oscillator is used for timing this real-time part of the start-up time. The number of WDT Oscillator cycles used for each time-out is shown in the table below. The frequency of the Watchdog Oscillator is voltage dependent as shown in Typical Characteristics – TA = -40°C to 85°C. The device is shipped with CKSEL = “0001” and SUT = “10” (1MHz Internal RC Oscillator, slowly rising power).

Table 2. Number of Watchdog Oscillator Cycles
Typical Time-out (VCC = 5.0V) Typical Time-out (VCC = 3.0V) Number of Cycles
4.1ms 4.3ms 4K (4,096)
65ms 69ms 64K (65,536)