TC820 datasheet(16/28 Pages) MICROCHIP

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TC820

DS21476B-page 16

4.2.1

EOC/HOLD

EOC/HOLD is a dual purpose, bi-directional 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 dis-

play 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 4-1. The output drive current is only a

few microAmps, so EOC/HOLD can easily be over-

driven by an open collector logic gate, as well as a FET,

bipolar transistor, 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.

FIGURE 4-1:

EOC/HOLD PIN

4.2.2

OVER RANGE (OR),

UNDER RANGE (UR)

The OR output will be high when the analog input sig-

nal is greater than full scale (3999 counts). The UR out-

put 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 under ranged or

over ranged conversion occurs (Figure 4-2).

FIGURE 4-2:

GENERATING UNDER

RANGE AND OVER

RANGE PULSES

4.2.3

VDISP

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

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

nally to DGND, providing a typical LCD drive voltage of

5VP-P. The 44-pin package includes a separate VDISP

input for applications requiring a variable or tempera-

ture compensated LCD drive voltage. See the applica-

tions information for suggested circuits.

EOC/HOLD

Display

Update

EOC

TC820

≈ 500kΩ

EOC/HOLD

TC820

*74HC132

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