Datasheet

51
7728G–AVR–06/10
ATtiny87/ATtiny167
Figure 6-3. MCU Start-up, RESET Extended Externally
6.1.4 External Reset
An External Reset is generated by a low level on the RESET
pin. Reset pulses longer than the
minimum pulse width (see Table 22-3 on page 246) will generate a reset, even if the clock is
not running. Shorter pulses are not guaranteed to generate a reset. When the applied signal
reaches the Reset Threshold Voltage – V
RST
– on its positive edge, the delay counter starts
the MCU after the Time-out period – t
TOUT
– has expired. The External Reset can be disabled
by the RSTDISBL fuse, see Table 21-4 on page 226.
Figure 6-4. External Reset During Operation
6.1.5 Brown-out Detection
ATtiny87/167 has an On-chip Brown-out Detection (BOD) circuit for monitoring the Vcc level
during operation by comparing it to a fixed trigger level. The trigger level for the BOD can be
selected by the BODLEVEL Fuses (See ”BODLEVEL Fuse Coding” on page 247.). The trigger
level has a hysteresis to ensure spike free Brown-out Detection. The hysteresis on the detec-
tion level should be interpreted as V
BOT
+
=V
BOT
+V
HYST
/2andV
BOT
–
=V
BOT
-V
HYST
/2.
When the BOD is enabled, and Vcc decreases to a value below the trigger level (V
BOT
–
in Fig-
ure 6-5), the Brown-out Reset is immediately activated. When Vcc increases above the trigger
level (V
BOT
+
in Figure 6-5), the delay counter starts the MCU after the Time-out period t
TOUT
has expired.
The BOD circuit will only detect a drop in Vcc if the voltage stays below the trigger level for lon-
ger than t
BOD
given in Table 22-6 on page 247.
V
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
RST
V
POR
CC
V
CCRR
CC