Rainbow Electronics AT89LP216 - Manual (Page 14)

14
3621A–MICRO–6/06
AT89LP216 [Preliminary]
10.2 Brown-out Reset
The AT89LP216 has an on-chip Brown-out Detection (BOD) circuit for monitoring the V
CC
level
during operation by comparing it to a fixed trigger level. The trigger level for the BOD is nomi-
nally 2.2V. The purpose of the BOD is to ensure that if V
CC
fails or dips while executing at speed,
the system will gracefully enter reset without the possibility of errors induced by incorrect execu-
tion. A BOD sequence is shown in
Figure 10-3 . When V
CC
decreases to a value below the
trigger level V
BOD
, the internal reset is immediately activated. When V
CC
increases above the
trigger level, the start-up timer releases the internal reset after the specified time-out period has
expired (
Table 10-1 ). The Brown-out Detector must be enabled by setting the BOD Enable Fuse.
( See “User Configuration Fuses” on page 71. ).
Figure 10-3. Brown-out Detector Reset
10.3 External Reset
The P1.3/RST pin can function as either an active-LOW reset input or as a digital general-
purpose I/O, P1.3. The Reset Pin Enable Fuse, when set to “1”, enables the external reset input
function on P1.3. (
See “User Configuration Fuses” on page 71. ). When cleared, P1.3 may be
used as an input or output pin. When configured as a reset input, the pin must be held low for at
least two clock cycles to trigger the internal reset.
Note: During a power-up sequence, the fuse selection is always overridden and therefore the pin will
always function as a reset input. An external circuit connected to this pin should not hold this
pin LOW during a power-on sequence as this will keep the device in reset until the pin tran-
sitions high. After the power-up delay, this input will function either as an external reset input or
as a digital input as defined by the fuse bit. Only a power-up reset will temporarily override the
selection defined by the reset fuse bit. Other sources of reset will not override the reset fuse bit.
P1.3/RST also serves as the In-System Programming (ISP) enable. ISP is enabled when the
external reset pin is held low. When the reset pin is disabled by the fuse, ISP may only be entered
by pulling P1.3 low during power-up.
Table 10-1. Start-up Timer Settings
SUT Fuse 1 SUT Fuse 0 Clock Source t
SUT
(± 5%) µs
0 0
Internal RC/External Clock 16
Crystal Oscillator 1024
0 1
Internal RC/External Clock 512
Crystal Oscillator 2048
1 0
Internal RC/External Clock 1024
Crystal Oscillator 4096
1 1
Internal RC/External Clock 4096
Crystal Oscillator 16384
V
CC
Time-out
VPOR
Internal
Reset
tSUT
VBOD