3-41

Zero-Crossing Flip-Flop

Figure 4 shows the problem that the zero-crossing F/F is

designated to solve.

The integrator output is approaching the zero-crossing point

where the count will be latched and the reading displayed.

For a 20,000 count instrument, the ramp is changing

approximately 0.50mV per clock pulse (10V Max integrator

output

divided

by

20,000

counts).

The

clock

pulse

feedthrough superimposed upon this ramp would have to be

less than 100mV peak to avoid causing significant errors.

The flip-flop interrogates the data once every clock pulse

after the transients of the previous clock pulse and half-clock

pulse have died down. False zero-crossings caused by clock

pulses are not recognized. Of course, the flip-flop delays the

true zero-crossing by one count in every instance, and if a

correction were not made, the display would always be one

count too high. Therefore, the counter is disabled for one

clock pulse at the beginning of phase 3. This one count

delay compensates for the delay of the zero crossing flip-

flop, and allows the correct number to be latched into the

display. Similarly, a one count delay at the beginning of

phase 1 gives an overload display of 0000 instead of 0001.

No delay occurs during phase 2, so that true ratiometric

readings result.

Detailed Description

DIGITAL SECTION

The 71C03 includes several pins which allow it to operate

conveniently in more sophisticated systems. These include:

4-1/2 / 3-1/2 (Pin 2)

When high (or open) the internal counter operates as a full

requiring 40,002 counts. When held low, the least significant

decade is cleared and the clock is fed directly into the next

decade. A measurement cycle now requires only 4,0002

clock pulses. All 5 digit drivers are active in either case, with

RUN/HOLD (Pin 4)

When high (or open) the A/D will free-run with equally

spaced measurement cycles every 40,0002/4,002 clock

pulses. If taken low, the converter will continue the full mea-

surement cycle that it is doing and then hold this reading as

long as Pin 4 is held low. A short positive pulse (greater then

300ns) will now initiate a new measurement cycle beginning

with up to 10,001/1,001 counts of auto zero. Of course if the

pulse

occurs

before

the

full

measurement

cycle

(40,002/4,002 counts) is completed, it will not be recognized

and the converter will simply complete the measurement it is

doing. An external indication that full measurement cycle

has been completed is that the first STROBE pulse (see

below) will occur 101/11 counts after the end of this cycle.

Thus, if RUN/HOLD is low and has been low for at least

101/11 counts, converter is holding and ready to start a new

measurement when pulsed high.

STROBE (Pin 18)

This is a negative-going output pulse that aids in transferring

the BCD data to external latches, UARTs or microproces-

sors. There are 5 negative-going STROBE pulses that occur

COUNTS

PHASE I

PHASE II

PHASE III

10,001

10,000

20,001

1,001

1,000

2,001

FIGURE 3. CONVERSION TIMING

POLARITY

DETECTED

ZERO CROSSING

OCCURS

ZERO CROSSING

DETECTED

DEINT PHASE III

INT PHASE II

AZ PHASE I

AZ

INTEGRATOR

OUTPUT

CLOCK

INTERNAL LATCH

BUSY OUTPUT

NUMBER OF COUNTS TO ZERO CROSSING

AFTER ZERO CROSSING,

ANALOG SECTION WILL

BE IN AUTOZERO

CONFIGURATION

FIGURE 4. INTEGRATOR OUTPUT NEAR ZERO-CROSSING

TRUE ZERO

CROSSING

CLOCK

PULSE

FEEDTHROUGH

FALSE ZERO

CROSSING

ICL8052/ICL71C03, ICL8068/ICL71C03