ATTINY87_14 Datasheet, PDF (31/261 Pages) ATMEL Corporation – 8-bit AVR Microcontroller with 8K/16K Bytes In-System

4.2.5
Low-frequency Crystal Oscillator
To use a 32.768kHz watch crystal as the clock source for the device, the low-frequency crystal oscillator must be selected by
setting CKSEL fuses or CSEL field as shown in Table 4-1 on page 26. The crystal should be connected as shown in Figure
4-3. Refer to the 32.768 kHz crystal oscillator application note for details on oscillator operation and how to choose
appropriate values for C1 and C2.
The 32.768kHz watch crystal oscillator can be used by the asynchronous timer if the (high-frequency) crystal oscillator is not
running or if the external clock is not enabled (Section 4.3.3 “Enable/Disable Clock Source” on page 33). The asynchronous
timer is then able to start itself this low-frequency crystal oscillator.
Figure 4-3. Low-frequency Crystal Oscillator Connections
C1 = 12 to 22pF
32.768KHz
C2 = 12 to 22pF
XTAL2
XTAL1
GND
12 to 22pF capacitors may be necessary if parasitic
impedance (pads, wires and PCB) is very low.
When this oscillator is selected, start-up times are determined by the SUT fuses or by CSUT field as shown in Table 4-8.
Table 4-8. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
SUT1..0⁽¹⁾
CSUT1..0⁽²⁾
Start-up Time from
Power-down/save
Additional Delay from
Reset (Vcc = 5.0V)
Recommended usage
00
1K(1024)CK⁽³⁾
4.1ms
Fast rising power or BOD enabled
01
1K(1024)CK⁽³⁾
65ms
Slowly rising power
10
32K(32768)CK
65ms
Stable frequency at start-up
11
Reserved
Notes: 1. Flash fuse bits.
2. CLKSELR register bits.
3. These options should only be used if frequency stability at start-up is not important for the application.
4.2.6
External Clock
To drive the device from this external clock source, CLKI should be driven as shown in Figure 4-4. To run the device on an
external clock, the CKSEL Fuses or CSEL field must be programmed as shown in Table 4-1 on page 26.
Figure 4-4. External Clock Drive Configuration
~
External
Clock
Signal
(XTAL2)
(CLKO)
CLKI
(XTAL1)
GND
ATtiny87/ATtiny167 [DATASHEET]
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7728H–AVR–03/14