MM74C923 20-40-60 Key Encoder

Use a MM74C923 to encode 20,40 or 60 keys on a 7 bits data, 1 bit enable output

Theory of Operation

The MM74C922/MM74C923 Keyboard Encoders implement all the logic necessary to interface a 16 or 20 SPST
key switch matrix to a digital system. The encoder will convert 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
the oscillator capacitor, COSE, and the key bounce mask capacitor, CMSK. Thus, the MM74C922/MM74C923’s performance 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 otherwise 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 combination 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.