M494 datasheet(7/22 Pages) STMICROELECTRONICS

7 / 22 page

DISPLAY

DRIVE

640

µs

(

2ms)

MUX PERIOD

(8ms)

Figure 5

FUNCTIONAL DESCRIPTION (continued)

Display, Keyboard and Data Multiplexing

Logic is integrated on the chip to provide the mul-

tiplexing between the display, keyboard and data

inputs. In the On state and with the Data Hand-

shake pin at logic HI as an input the display and

keyboard are muxed together. See Figure 5. Each

column output goes to logic LO in sequence and

the row inputs are scanned for a key closure. In

chronological order across the total mux. period

there is : initialisation, scan, decision and display

periods.

The Data Handshake pin has a complex logical

function. It has two modes of operation : as a hand

shake I/O line to a

µP and as an output line to the

P to signal that the M494 is in the Off state. In order

to achieve this function careful signal timing is

required both internally and externally to the chip.

See Figure 6. When the device is in the OFF state

the Data Handshake pin is used to signal this

condition to the

µP by being pulled LO.

DATA

O/I

I/O

O/I

I/O

(EOT = END OF

TRANSMISSION)

K/B AND RC I/P

DISABLED TILL

EOT EXECYTION

DATA

HANDSHAKE

D0 - D4

EOE

(END OF TRANSMISSION)

NOT TO SCALE

DATA HANDSHAKE

PIN CONFIGURATION

RTS

CTS

O/P

CTS

Figure 6 : Data-input-timing

° Symbol

Parameter

Min.

Typ.

Max.

Unit

1tRTS

Request to Send Pulse (RTS)

µs

2tCTS

Clear to Send (CTS)

512

µs

3tO/P

Pin Output Configuration

4tHS

Handshake Time

µs

5tS

Data Set up Time

128

µs

6tH

Data Hold Time

128

256

µs

7tJ

Synchronization Jitter

8tCR

CTS to next RTS Pulse

Note : Oscillator Clock = 500kHz

M494

7/22

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