ICL7135 Datasheet, PDF (6/15 Pages) Intersil Corporation – 4 1/2 Digit, BCD Output, A/D Converter

ICL7135
Digital Section
Figure 5 shows the Digital Section of the ICL7135. The
ICL7135 includes several pins which allow it to operate
conveniently in more sophisticated systems. These include:
Run/HOLD (Pin 25)
When high (or open) the A/D will free-run with equally
spaced measurement cycles every 40,002 clock pulses. If
taken low, the converter will continue the full measurement
cycle that it is doing and then hold this reading as long as
R/H is held low. A short positive pulse (greater than 300ns)
will now initiate a new measurement cycle, beginning with
between 1 and 10,001 counts of auto zero. If the pulse
occurs before the full measurement cycle (40,002 counts) is
completed, it will not be recognized and the converter will
simply complete the measurement it is doing. An external
indication that a full measurement cycle has been completed
is that the first strobe pulse (see below) will occur 101 counts
after the end of this cycle. Thus, if Run/HOLD is low and has
been low for at least 101 counts, the converter is holding and
ready to start a new measurement when pulsed high.
STROBE (Pin 26)
This is a negative going output pulse that aids in transferring
the BCD data to external latches, UARTs, or
microprocessors. There are 5 negative going STROBE
pulses that occur in the center of each of the digit drive
pulses and occur once and only once for each measurement
cycle starting 101 clock pulses after the end of the full
measurement cycle. Digit 5 (MSD) goes high at the end of
the measurement cycle and stays on for 201 counts. In the
center of this digit pulse (to avoid race conditions between
changing BCD and digit drives) the first STROBE pulse goes
negative for ¹/₂ clock pulse width. Similarly, after digit 5, digit
4 goes high (for 200 clock pulses) and 100 pulses later the
STROBE goes negative for the second time. This continues
through digit 1 (LSD) when the fifth and last STROBE pulse
is sent. The digit drive will continue to scan (unless the
previous signal was overrange) but no additional STROBE
pulses will be sent until a new measurement is available.
BUSY (Pin 21)
BUSY goes high at the beginning of signal integrate and
stays high until the first clock pulse after zero crossing (or
after end of measurement in the case of an overrange). The
internal latches are enabled (i.e., loaded) during the first
clock pulse after busy and are latched at the end of this clock
pulse. The circuit automatically reverts to auto-zero when not
BUSY, so it may also be considered a (Zl + AZ) signal. A
very simple means for transmitting the data down a single
wire pair from a remote location would be to AND BUSY with
clock and subtract 10,001 counts from the number of pulses
received - as mentioned previously there is one “NO-count”
pulse in each reference integrate cycle.
OVERRANGE (Pin 27)
This pin goes positive when the input signal exceeds the
range (20,000) of the converter. The output F/F is set at the
end of BUSY and is reset to zero at the beginning of
reference integrate in the next measurement cycle.
UNDERRANGE (Pin 28)
This pin goes positive when the reading is 9% of range or
less. The output F/F is set at the end of BUSY (if the new
reading is 1800 or less) and is reset at the beginning of
signal integrate of the next reading.
POLARlTY (Pin 23)
This pin is positive for a positive input signal. It is valid even
for a zero reading. In other words, +0000 means the signal is
positive but less than the least significant bit. The converter
can be used as a null detector by forcing equal frequency of
(+) and (-) readings. The null at this point should be less than
0.1 LSB. This output becomes valid at the beginning of
reference integrate and remains correct until it is revalidated
for the next measurement.
ANALOG
SECTION
V ^{+ POLARITY}
D5
D4
D3
D2
D1
11
23
12
MSB
17
18
19
MULTIPLEXER
20
13 B1
LSB ^{14 B2}
15 B4
16 B8
POLARITY
FF
LATCH LATCH ^LATCH LATCH LATCH
ZERO
CROSS.
DET.
COUNTERS
CONTROL LOGIC
24
22
25
27
28
26
21
DIGITAL
GND
CLOCK RUN/ OVER UNDER _STROBE BUSY
IN
HOLD RANGE RANGE
FIGURE 5. DIGITAL SECTION OF THE ICL7135
6
FN3093.3