TC7129
DS21459E-page 16 2002-2012 Microchip Technology Inc.
Figure 4-15: Digital Ground (DGND) and
Common Outputs.
4.14 Low Battery
The low battery annunciator turns on when supply volt-
age between V– and V+ drops below 6.8V. The internal
zener diode has a threshold of 6.3V. When the supply
voltage drops below 6.8V, the transistor tied to V– turns
off pulling the “Low Battery” point high.
4.15 Sequence and Results Counter
A sequence counter and associated control logic pro-
vide signals that operate the analog switches in the
integrator section. The comparator output from the inte-
grator gates the results counter. The results counter is
a six-section up/down decade counter that holds the
intermediate results from each successive integration.
4.16 Overrange and Underrange
Outputs
When the results counter holds a value greater than
±19,999, the DP
4
/OR output (Pin 20) is driven high.
When the results counter value is less than ±1000, the
DP
3
/UR output (Pin 21) is driven high. Both signals are
valid on the falling edge of LATCH
/HOLD (L/H) and do
not change until the end of the next conversion cycle.
The signals are updated at the end of each conversion,
unless the L
/H input (Pin 22) is held high. Pins 20 and
21 can also be used as inputs for external control of
decimal points 3 and 4. Figure 4-14 shows a schematic
of the input/output nature of these pins.
4.17 LATCH/Hold
The L/H output goes low during the last 100 cycles of
each conversion. This pulse latches the conversion
data into the display driver section of the TC7129. This
pin can also be used as an input. When driven high, the
display will not be updated; the previous reading is
displayed. When driven low, the display reading is not
latched; the sequence counter reading will be
displayed. Since the counter is counting much faster
than the backplanes are being updated, the reading
shown in this mode is somewhat erratic.
4.18 Display Driver
The TC7129 drives a triplexed LCD with three back-
planes. The LCD can include decimal points, polarity
sign and annunciators for continuity and low battery.
Figure 4-16 shows the assignment of the display
segments to the backplanes and segment drive lines.
The backplane drive frequency is obtained by dividing
the oscillator frequency by 1200. This results in a back-
plane drive frequency of 100 Hz for 60 Hz operation
(120 kHz crystal) and 83.3 Hz for 50 Hz operation
(100 kHz crystal).
Backplane waveforms are shown in Figure 4-17.
These appear on outputs BP
1
, BP
2
, BP
3
(pins 16, 17
and 18). They remain the same, regardless of the
segments being driven.
Other display output lines (pins 4 through 15) have
waveforms that vary depending on the displayed
values. Figure 4-18 shows a set of waveforms for the
A, G, D outputs (pins 5, 8, 11 and 14) for several
combinations of “ON” segments.
The ANNUNCIATOR DRIVE output (pin 3) is a square
wave, running at the backplane frequency (100 Hz or
83.3 Hz) with a peak-to-peak voltage equal to DGND
voltage. Connecting an annunciator to pin 3 turns it on;
connecting it to its backplane turns it off.
+
–
12 µA
P
TC7129
Logic
Section
5V
3.2V
N
N
V+
V–
COM
DGND
24
28
36
23
