
176
42073A-MCU Wireless-02/13
ATmega2564/1284/644RFR2
To ensure sufficient startup time, the device issues an internal reset with a time-out
delay (tTOUT) after the device reset is released by all other reset sources. Section
delay (tTOUT) is timed from the Watchdog Oscillator and the number of cycles in the
delay is set by the SUTx and CKSELx fuse bits. The selectable delays are shown in
Table 11-2. Number of Watchdog Oscillator Cycles
Typ Time-out
Number of Cycles
0 ms
0
4.0 ms
512
64 ms
8K (8,192)
Main purpose of the delay is to keep the AVR in reset until it is supplied with a stable
VDEVDD. The delay will not monitor the actual voltage and it will be required to select a
delay longer than the DEVDD rise time. If this is not possible, an internal or external
Brown-Out Detection (BOD) circuit should be used. A BOD circuit will ensure sufficient
VDEVDD before it releases the reset, and the time-out delay can be disabled. Disabling
the time-out delay without utilizing a Brown-Out Detection circuit is not recommended.
The oscillator is required to oscillate for a minimum number of cycles before the clock is
considered stable. An internal ripple counter monitors the oscillator output clock, and
keeps the internal reset active for a given number of clock cycles. The reset is then
released and the device will start to execute. The recommended oscillator start-up time
is dependent on the clock type, and varies from 6 cycles for an externally applied clock
to 32K cycles for a low frequency crystal.
The start-up sequence for the clock includes both the time-out delay and the start-up
time when the device starts up from reset. When starting up from Power-save or Power-
down mode, DEVDD is assumed to be at a sufficient level and only the start-up time is
included.
11.4 Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an approximate 16 MHz clock. The RC
oscillator is voltage and temperature dependent, but can be very accurately calibrated
This clock may be selected as the system clock by programming the CKSEL Fuses as
components. During reset, hardware loads the pre-programmed calibration value into
the OSCCAL Register and thereby automatically calibrates the RC Oscillator. The
accuracy of this calibration is shown as Factory calibration in section
"Clockpage 180) from Software, it is possible to get a higher calibration accuracy than by
using the factory calibration. The accuracy of this calibration is shown as User