Specifications

35
3706C–MICRO–2/11
AT89LP3240/6440
meet the minimum system requirements before the device exits reset and starts normal opera-
tion. The RST
pin may be held low externally until these conditions are met.
7.2 Brown-out Reset
The AT89LP3240/6440 has an on-chip Brown-out Detection (BOD) circuit for monitoring the V
DD
level during operation by comparing it to a fixed trigger level. The trigger level V
BOD
for the BOD
is nominally 2.0V. The purpose of the BOD is to ensure that if V
DD
fails or dips while executing at
speed, the system will gracefully enter reset without the possibility of errors induced by incorrect
execution. A BOD sequence is shown in Figure 7-3. When V
DD
decreases to a value below the
trigger level V
BOD
, the internal reset is immediately activated. When V
DD
increases above the
trigger level plus about 200 mV of hysteresis, the start-up timer releases the internal reset after
the specified time-out period has expired (Table 7-1). The Brown-out Detector must be enabled
by setting the BOD Enable Fuse. (See “User Configuration Fuses” on page 164.)
Figure 7-3. Brown-out Detector Reset
The AT89LP3240/6440 allows for a wide V
DD
operating range. The on-chip BOD may not be suf-
ficient to prevent incorrect execution if V
BOD
is lower than the minimum required V
DD
range, such
as when a 3.6V supply is coupled with high frequency operation. In such cases an external
Brown-out Reset circuit connected to the RST
pin may be required.
7.3 External Reset
The P4.2/RST pin ca n function as either an active-LOW reset input or as a digital general-
purpose I/O, P4.2. The Reset Pin Enable Fuse, when set to “1”, enables the external reset input
function on P4.2. (See “User Configuration Fuses” on page 164.) When cleared, P4.2 may be
used as an input or output pin. When configured as a reset input, the pin must be held low for at
le
ast two clock cycles to trigger the internal reset. The RST pin includes an on-chip pull-up resis-
tor tied to V
DD
. The pull-up is disabled when the pin is configured as P4.2.
Table 7-1. Start-up Timer S ettings
SUT Fuse 1 SUT Fuse 0 Clock Source t
SUT
(± 5%) µs
00
Internal RC/External Clock 16
Crystal Oscillator 1024
01
Internal RC/External Clock 512
Crystal Oscillator 2048
10
Internal RC/External Clock 1024
Crystal Oscillator 4096
11
Internal RC/External Clock 4096
Crystal Oscillator 16384
V
DD
Time-out
V
POR
Internal
Reset
t
SUT
V
BOD