Rainbow Electronics ATtiny28L - Manual (Page 15)

15
ATtiny28L/V
1062E–10/01
Note: 1. Due to limited number of clock cycles in the start-up period, it is recommended that
ceramic resonator be used.
This table shows the start-up times from reset. From Power-down mode, only the clock
counting part of the start-up time is used. The Watchdog oscillator is used for timing the
real-time part of the start-up time. The number WDT oscillator cycles used for each
time-out is shown in Table 6.
The frequency of the Watchdog oscillator is voltage-dependent, as shown in the section
“Typical Characteristics” on page 55.
The device is shipped with CKSEL = 0010.
Power-on Reset A Power-on Reset (POR) pulse is generated by an on-chip detection circuit. The detec-
tion level is nominally 1.4V. The POR is activated whenever V
CC
is below the detection
level. The POR circuit can be used to trigger the start-up reset, as well as detect a fail-
ure in supply voltage.
The Power-on Reset (POR) circuit ensures that the device is reset from power-on.
Reaching the Power-on Reset threshold voltage invokes a delay counter, which deter-
mines the delay for which the device is kept in RESET after V
CC
rise. The time-out
period of the delay counter can be defined by the user through the CKSEL fuses. The
different selections for the delay period are presented in Table 5. The RESET signal is
activated again, without any delay, when the V
CC
decreases below detection level. See
Figure 16.
If the built-in start-up delay is sufficient, RESET can be connected to VCC directly or via
an external pull-up resistor. By holding the RESET pin low for a period after V
CC
has
been applied, the Power-on Reset period can be extended. Refer to Figure 17 for a tim-
ing example of this.
0100 Internal RC Oscillator 6 CK
0011 Internal RC Oscillator 4.2 ms + 6 CK
0010 Internal RC Oscillator 67 ms + 6 CK
0001 External Clock 6 CK
0000 External Clock 4.2 ms + 6 CK
Table 6. Number of Watchdog Oscillator Cycles
Time-out Number of Cycles
4.2 ms 1K
67 ms 16K
Table 5. ATtiny28 Clock Options and Start-up Time (Continued)
CKSEL3..0 Clock Source Start-up Time at 2.7V