Asynchronous Data Entry Onto Bus (MM74C922)

TLF6037 – 12

Outputs are in TRI-STATE until key is pressed then data is placed on bus

When key is released outputs return to TRI-STATE

Expansion to 32 Key Encoder (MM74C922)

TLF6037 – 13

Theory of Operation

The MM74C922MM74C923 Keyboard Encoders imple-

ment all the logic necessary to interface a 16 or 20 SPST

key switch matrix to a digital system The encoder will con-

vert

a

key

switch

closer

to

a

4(MM74C922)

or

5(MM74C923) bit nibble The designer can control both the

keyboard scan rate and the key debounce period by altering

formance can be optimized for many keyboards

The keyboard encoders connect to a switch matrix that is 4

rows by 4 columns (MM74C922) or 5 rows by 4 columns

(MM74C923) When no keys are depressed the row inputs

are pulled high by internal pull-ups and the column outputs

sequentially output a logic ‘‘0’’ These outputs are open

drain and are therefore low for 25% of the time and other-

wise off The column scan rate is controlled by the oscillator

input which consists of a Schmitt trigger oscillator a 2-bit

counter and a 2 – 4-bit decoder

When a key is depressed key 0 for example nothing will

happen when the X1 input is off since Y1 will remain high

When the X1 column is scanned X1 goes low and Y1 will go

low This disables the counter and keeps X1 low Y1 going

low also initiates the key bounce circuit timing and locks out

the other Y inputs The key code to be output is a combina-

tion of the frozen counter value and the decoded Y inputs

Once the key bounce circuit times out the data is latched

and the Data Available (DAV) output goes high

If during the key closure the switch bounces Y1 input will

go high again restarting the scan and resetting the key

bounce circuitry The key may bounce several times but as

soon as the switch stays low for a debounce period the

closure is assumed valid and the data is latched

A key may also bounce when it is released To ensure that

the encoder does not recognize this bounce as another key

closure the debounce circuit must time out before another

closure is recognized

The two-key roll-over feature can be illustrated by assuming

a key is depressed and then a second key is depressed

Since all scanning has stopped and all other Y inputs are

disabled the second key is not recognized until the first key

is lifted and the key bounce circuitry has reset

The output latches feed TRI-STATE which is enabled when

the Output Enable (OE) input is taken low

6