Datasheet

30
7728G–AVR–06/10
ATtiny87/ATtiny167
Figure 4-3. Low-frequency Crystal Oscillator Connections
When this oscillator is selected, start-up times are determined by the SUT fuses or by CSUT
field as shown in Table 4-8.
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 pro-
grammed as shown in Table 4-1 on page 25.
Figure 4-4. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses or CSUT
field as shown in Table 4-9.
This external clock can be used by the asynchronous timer if the high or low frequency Crystal
Oscillator is not running (See ”Enable/Disable Clock Source” on page 32.). The asynchronous
timer is then able to enable this input.
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)
4.1 ms Fast rising power or BOD enabled
01 1K (1024) CK
(3)
65 ms Slowly rising power
10 32K (32768) CK 65 ms Stable frequency at start-up
11 Reserved
XTAL2
XTAL1
GND
C1=12-22 pF
32.768 KHz
12-22 pF capacitors may be necessary if parasitic
impedance (pads, wires & PCB) is very low.
C2=12-22 pF
(XTAL2)
(CLKO)
CLKI
(XTAL1)
GND
~
External
Clock
Signal