Datasheet

26
7728G–AVR–06/10
ATtiny87/ATtiny167
The various choices for each clocking option are given in the following sections.
When the CPU wakes up from Power-down or Power-save, or when a new clock source is
enabled by the dynamic clock switch circuit, 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 an additional delay allowing the power to reach a sta-
ble level before commencing normal operation. The Watchdog Oscillator is used for timing this
real-time part of the start-up sequence. The number of WDT Oscillator cycles used for each
time-out is shown in Table 4-2.
4.2.1 Default Clock Source
At reset, the CKSEL and SUT fuse settings are copied into the CLKSELR register. The device
will then use the clock source and the start-up timings defined by the CLKSELR bits
(CSEL3..0 and CSUT1:0).
The device is shipped with CKSEL Fuses = 0010
b
, SUT Fuses = 10
b
, and CKDIV8 Fuse pro-
grammed. The default clock source setting is therefore the Internal RC Oscillator running at 8
MHz with the longest start-up time and an initial system clock divided by 8. This default setting
ensures that all users can make their desired clock source setting using an In-System or
High-voltage Programmer. This set-up must be taken into account when using ISP tools.
4.2.2 Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an approximate 8.0 MHz clock. Though voltage
and temperature dependent, this clock can be accurately calibrated by the user. See Table
22-1 on page 245 and Section 24.7 “Internal Oscillator Speed” on page 265 for more details.
If selected, it can operate without external components. At reset, hardware loads the pre-pro-
grammed calibration value into the OSCCAL Register and thereby automatically configuring
the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in Table
22-1 on page 245.
By adjusting the OSCCAL register in software, see “OSCCAL – Oscillator Calibration Regis-
ter” on page 37, 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 Table 22-1 on page
245.
The Watchdog Oscillator will still be used for the Watchdog Timer and for the Reset Time-out
even when this Oscillator is used as the device clock. For more information on the pre-pro-
grammed calibration value, see the section “Calibration Byte” on page 228.
Table 4-2. Number of Watchdog Oscillator Cycles
Typ. Time-out (Vcc = 5.0V) Typ. Time-out (Vcc = 5.0V) Number of Cycles
4.1 ms 4.3 ms 512
65 ms 69 ms 8K (8,192)