TC820CPL datasheet(15/22 Pages) TELCOM

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TELCOM SEMICONDUCTOR, INC.

TC820

3-3/4 A/D CONVERTER WITH

FREQUENCY COUNTER

AND LOGIC PROBE

Additional Features

The TC820 is available in 40-pin and 44-pin packages.

Several additional features are available in the 44-pin pack-

age.

EOC/HOLD

EOC/HOLD is a dual-purpose, bidirectional pin. As an

output, this pin goes low for 10 clock cycles at the end of each

conversion. This pulse latches the conversion data into the

display driver section of the TC820.

EOC/HOLD can be used to hold (or "freeze") the dis-

play. Connecting this pin to VDD inhibits the display update

process. Conversions will continue, but the display will not

change. EOC/HOLD will hold the display reading for either

analog-to-digital or frequency measurements.

The input/output structure of the EOC/HOLD pin is

shown in Figure 10. The output drive current is only a few

microAmps, so EOC/HOLD can easily be overdriven by an

open-collector logic gate, as well as a FET, bipolar transis-

tor, or mechanical switch. When used as an output, EOC/

HOLD will have a slow rise and fall time due to the limited

output current drive. A CMOS Schmitt trigger buffer is

recommended.

Connecting the LOGIC input to VDD places the TC820

in the logic probe mode. In this mode, the DP0/LO and DP1/

HI inputs control the LCD "low" and "high" annunciators

directly. When DP1/HI is connected to VDD, the "high"

annunciator will turn on. When DP0/LO is connected to VDD,

the "low" annunciator and the buzzer will turn on. The

internal pull-downs on these pins are disabled when the

logic probe function is selected.

These inputs have CMOS logic switching thresholds.

For optimum performance as a logic probe, external level

shifters are recommended. See the applications section for

details.

BUZ IN

This input controls the TC820 on-chip buzzer driver.

Connecting BUZ IN to VDD will turn the buzzer on. There is

an external pull-down to DGND. BUZ IN can be used with

external circuitry to provide additional functions, such as a

fast, audible continuity indication.

Figure 10. EOC/HOLD Pin Schematic

Ω

500 k

≈

EOC/HOLD

DISPLAY

UPDATE

EOC

TC820

Overrange (OR), Underrange (UR)

The OR output will be high when the analog input signal

is greater than full scale (3999 counts). The UR output will

be high when the display reading is 380 counts or less.

The OR and UR outputs can be used to provide an auto-

ranging meter function. By logically ANDing these outputs

with the inverted EOC/HOLD output, a single pulse will be

generated each time an underranged or overranged conver-

sion occurs (Figure 11).

VDISP

The VDISP input sets the peak-to-peak LCD drive volt-

age. In the 40-pin package, VDISP is connected internally to

DGND, providing a typical LCD drive voltage of 5VP-P. The

44-pin package includes a separate VDISP input for applica-

tions requiring a variable or temperature-compensated LCD

drive voltage. See the applications information for sug-

gested circuits.

EOC/HOLD

TC820

74HC132

Figure 11. Generating Underrange and Overrange Pulses

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