3-49

Interfacing with UARTs and

Microprocessors

Figure 15 shows a very simple interface between a free-run-

ning 8068/8052/71C03A and a UART. The five STROBE

pulses start the transmission of the five data words. The digit

5 word is 0000XXXX, digit 4 is 1000XXXX, digit 3 is

0100XXXX, etc. Also, the polarity is transmitted indirectly by

using it to drive the Even Parity Enable Pin (EPE). If EPE of

the receiver is held low, a parity flag at the receiver can be

decoded as a positive signal, no flag as negative. A complex

arrangement is shown in Figure 14. Here the UART can

instruct the A/D to begin a measurement sequence by a

word on RRI. The Busy signal resets the Data Ready Reset

(DRR). Again STROBE starts the transmit sequence. A

quad 2 input multiplexer is used to superimpose polarity,

For correct operation it is important that the UART clock be

fast enough that each word is transmitted before the next

STROBE pulse arrives. Parity is locked into the UART at

load time but does not change in this connection during an

output stream.

Circuits to interface the 71C03(A) directly with three popular

microprocessors are shown in Figures 17, 18 and 19. The

main differences in the circuits are that the IM6100 with its

12-bit word capability can accept polarity, over-range, under-

range, 4 bits of BCD and 5 digits simultaneously where the

8080/8048 and the MC6800 groups with 8-bit words need to

have polarity, over-range and under-range multiplexed onto

the Digit 5 word - as in the UART circuits. In each case the

microprocessor can instruct the A/D when to begin a mea-

surement and when to hold this measurement.

Application Notes

FIGURE 12. CMOS OSCILLATOR

FIGURE 13. LM311 OSCILLATOR

FIGURE 14. GROSS OVERVOLTAGE PROTECTION CIRCUIT

R

37.5k

Ω

C

100pF

LM311

-

+

1

4

3

2

7

8

56k

Ω

1k

Ω

30k

Ω

16k

Ω

16k

Ω

390pF

+5V

0.22

µF

A2

+

-

A3

+

-

INTEG.

COMP.

A1

+

-

BUFFER

14

11

9

INT OUT

-INT IN

BUF OUT

10

-BUF IN

-1.2V

2

1

-15V

7

8

+15V

12

+INT IN

13

8052A/

INT.

REF.

6

3

+BUF IN

5

REF

OUT

300pF

COMP

OUT

100K

0.22

µF

2N5458

2N5461

S

D

S

D

8068A

REF

COMP

TO ICL71C03

NOTE #

DESCRIPTION

AnswerFAX

DOC. #

AN016

“Selecting A/D Converters”

9016

AN017

“The Integrating A/D Converter”

9017

AN018

“Do’s and Don’ts of Applying A/D

Converters”

9018

AN023

“Low Cost Digital Panel Meter Designs”

9023

AN028

“Build an Auto-Ranging DMM Using the

8052A / 7103A A/D Converter Pair,” by

Larry Goff

9028

ICL8052/ICL71C03, ICL8068/ICL71C03