Datasheet

© 2007 Microchip Technology Inc. DS21483D-page 17
TC9400/9401/9402
7.0 F/V POWER-ON RESET
In F/V mode, the TC9400 output voltage will occasion-
ally be at its maximum value when power is first
applied. This condition remains until the first pulse is
applied to F
IN
. In most frequency measurement
applications, this is not a problem because proper
operation begins as soon as the frequency input is
applied.
In some cases, however, the TC9400 output must be
zero at power-on without a frequency input. In such
cases, a capacitor connected from Pin 11 to V
DD
will
usually be sufficient to pulse the TC9400 and provide a
Power-on Reset (see Figure 7-1 (a) and (b)). Where
predictable power-on operation is critical, a more
complicated circuit, such as Figure 7-1 (b), may be
required.
FIGURE 7-1: Power-On Operation/Reset.
V
DD
14
11
1000 pF
Threshold
Detector
1kΩ
F
IN
V
DD
100 kΩ
1µF
3
4
8
6
F
IN
12516
V
CC
B R C
Q
V
SS
A
CLRA
CD4538
TC9400
(a) (b)
To TC9400