Datasheet

14
LTC690/LTC691
LTC694/LTC695
Low External Clock Input 1024 clks 4096 clks 512 clks 2048 clks
Low External Capacitor*
Floating or High Low 100ms 1.6 sec 50ms 200ms
Floating or High Floating or High 1.6 sec 1.6 sec 50ms 200ms
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
IMMEDIATELY
AFTER RESET
(Long Period)
1.6 sec
• C
70pF
200ms
• C
70pF
800ms
• C
70pF
NORMAL
(Short Period)
WATCHDOG TIME-OUT PERIOD
RESET ACTIVE TIME
LTC691
LTC695
400ms
• C
70pF
OSC INOSC SEL
Table 2. LTC691 and LTC695 Reset Active Time and Watchdog Time-Out Selections
Figure 13. The External Push-Button Reset
Figure 14. The External Push-Button Reset with the LTC1235
Push-Button Reset
The LTC690 family does not provide a logic input for direct
connection to a pushbutton. However, a push-button in
series with a 100Ω resistor connected to the RESET output
pin (Figure 13) provides an alternative for manual reset.
Connecting a 0.1µF capacitor to the RESET pin debounces
the push-button input.
The 100Ω resistor in series with the push-button is
required to prevent the ringing, due to the capacitance and
lead inductance, from pulling the RESET pins of the MPU
and LTC69X below ground.
If a dedicated pushbutton reset input is desired, the
LTC1235 is a good choice (Figure 14). It has all the
functions of the LTC695 and provides push-button reset
as an extra feature. Its push-button is internally debounced
and invokes the normal 200ms reset sequence. This
eliminates the need for the 100Ω resistor and 0.1µF
capacitor. It also provides a more consistent reset pulse.
5V
100Ω
V
CC
LTC690/LTC691
LTC694/LTC695
GND
690 F13
RESET
0.1µF
MPU
(e.g. 6805)
RESET
5V
V
CC
LTC1235
GND
690 F14
RESET
MPU
(e.g. 6805)
RESET
PBRST
*The nominal internal frequency is 10.24kHz. The nominal oscillator frequency with external capacitor is f
OSC
(Hz) =
184,000
C(pF) • 1025