4 calibrated internal rc oscillator, Atmega128rfa1 – Rainbow Electronics ATmega128RFA1 User Manual
Page 149
149
8266A-MCU Wireless-12/09
ATmega128RFA1
To ensure sufficient startup time, the device issues an internal reset with a time-out
delay (t
TOUT
) after the device reset is released by all other reset sources. Section
describes the start conditions for the internal reset. The
delay (t
TOUT
) 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
. The frequency of the Watchdog Oscillator is voltage dependent as
shown in section
"Typical Characteristics" on page 510
.
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
V
DEVDD
. 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
V
DEVDD
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
by the user. See chapter
"Clock Characteristics" on page 502
for more details. The device is shipped with the CKDIV8 Fuse and
the 1:2 system clock prescaler programmed. See section
for more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as
shown in
page 150. If selected, it will operate with no external
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
.
By changing the OSCCAL register (see
"OSCCAL – Oscillator Calibration Value" on
) 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
calibration in section
"Clock Characteristics" on page 502
.