31
7734Q–AVR–02/12
AT90PWM81/161
5.2.2.1
RC Oscillator calibration at Factory
The RC oscillator is calibrated at 3V, 25°C for an 8MHz target frequency with an Accuracy ±1%.
The corresponding value OSCAL (@Amb.) is stored in the signature row and automatically
loaded in the OSCAL register at reset.
The RC oscillator is monitored at 105°C or 125°C (versus Product version) with an accuracy
within ±5% limits.
5.2.3
128KHz Internal Oscillator
The 128kHz internal Oscillator is a low power Oscillator providing a clock of 128kHz. The fre-
quency is nominal at 3V and 25°C. This clock may be select as the system clock by
When this clock source is selected, start-up times are determined by the SUT Fuses or by CSUT
Notes:
1. Flash Fuse bits.
2. CLKSELR register bits.
5.2.4
Crystal Oscillator
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be con-
crystal or a ceramic resonator may be used.
C1 and C2 should always be equal for both crystals and resonators. The optimal value of the
capacitors depends on the crystal or resonator in use, the amount of stray capacitance, and the
electromagnetic noise of the environment. Some initial guidelines for choosing capacitors for
use with crystals are given in
Table 5-6. For ceramic resonators, the capacitor values given by
the manufacturer should be used.
Figure 5-2.
Crystal oscillator connections.
Table 5-5.
Start-up times for the 128kHz internal oscillator.
Start-up time from power-
down
Additional delay from
reset
Recommended
usage
00
6 CK
14CK
BOD enabled
01
6 CK
14CK + 4ms
Fast rising power
10
6 CK
14CK + 64ms
Slowly rising power
11
Reserved
XTAL2
XTAL1
GND
C2
C1