Design of single-pin keyboard interface circuit to save power

This design realizes the connection of a 4-row 4-column matrix keyboard to a single-chip microcomputer through one pin . It is an ideal choice when the number of pins is limited and has the advantage of saving energy. The 555 timer has an output current. In order to reduce power consumption, the circuit is designed as shown in Figure 1, and there is only leakage loss on the CMOS integrated circuit.
When a key is pressed, the 74HC14 Schmitt trigger forms an oscillator. The output of U1C is the feedback signal connected to the input of U1A and the time base capacitor C1, then an oscillation circuit is constructed to generate an output pulse. In the figure, SIL1, SIL2 and C1 determine the pulse oscillation period. Due to the different switching thresholds of CMOS and 555, the frequency selection of resistors is unstable in time or temperature. This paper introduces current control diodes D1~D10 to solve this problem. The SLI1 and SLI2 resistor networks determine the duty time respectively. Since there are 4 different duty times in one cycle, a small change in temperature will not generate an erroneous key signal. The single-chip microcomputer determines whether a key is pressed by querying the duty time and using the threshold value encoding and decoding.
When no key is pressed, the feedback circuit is open, and the input of U1A is set to 0 through R1. All inputs and outputs of the 74HC14 in the figure are 0V or VCC, so the leakage loss is low, the power supply works in an ideal state, and a pulse signal is generated when a key is pressed. When the microcontroller is not working, no key query is performed and it is in a dormant state.
In the figure, D11 and D12 realize temperature compensation. When the temperature change is not large, D11 and D12 can be omitted. ■


Figure 1 When a key is pressed, the 74HC14 Schmitt trigger forms an oscillator.