Interrupt-driven keyboard for MCS-51 uses few components

Designers of micro-controller based products that require a keypad for user data entry can select from dedicating an input line for each key, continuously polling the keypad’s x and y lines, or generating an interrupt whenever a user presses a key. Although conceptually simple, dedicating lines to a keypad can tie up most of the microcontroller’s I/O resources. Continuously polled keypads can burden the microprocessor’s resources and consume excessive amounts of battery power.

The third method, an interruptdriven keypad, offers several benefits. First, using interrupts frees the microcontroller to perform other tasks or to switch into an idling or power-down mode while awaiting the next key closure. Second, using interrupts helps reduce electromagnetic interference produced by continuously scanning the keypad’s lines. Figure 1 shows an interruptdriven keypad implementation that’s based on Atmel’s AT89C52 version of the popular MCS-51 family of microcontrollers. Here, the rows of a 16-key keypad, S1 through S16, implemented as a 4_4-key matrix connect to the lower nibble (P1.0 to P1.3) of IC1’s Port 1. The keypad’s columns connect to IC1’s Port 1 upper nibble (P1.4 to P1.7) and a network of four diodes (D1 through D4 and a 10kΩresistor, R9. The junction of R9 and the diodes’ anodes connects to Port Pin 3.2 and generates an interrupt whenever the user presses a key.

Initially, Port 1’s lower nibble sits high at logic one, and the upper nibble is grounded at logic zero, applying reverse bias to the diodes and pulling the Into signal high. Pressing a key applies forward bias to the diode corresponding to that row and causes Into to go low, generating an external interrupt to the microcontroller. Upon receiving an interrupt and after a 20msec softwaredebouncing interval, the microprocessor sequentially reads the row and column lines. Capacitor C1 provides a hardware-based debouncing interval of approximately 25msec.

In this design, the microcontroller’s software returns a binary-formatted input corresponding to the pressed key’s number as sensed at Port P1 (P1.0 to P1.3). As the commented assembly-language routine, available with the online version of this Design Idea at www.edn.com, explains, the software ignores invalid key combinations. Idle and power-down modes available in CHMOS (complementary high-density MOS) versions of the MCS-51 family save power and thus make these microcontrollers ideal choices for battery-operated devices. For example, a 5V, 12MHz Atmel AT89C52 consumes approximately 25mA in active mode, 6.5mA in idle mode, and only 100µA in powerdown mode. Any enabled interrupt can switch the microprocessor from idle to active modes.

However, recovery from powerdown mode to active mode normally requires a hardware reset—an apparent limitation of the MCS-51 microcontroller. However, an earlier Design Idea overcomes the problem and allows use of an interrupt-driven keypad even in hardware-reset-based systems (Reference 1).

Reference
1. Chrzaszcz, Jerzy, “Use 8051’s power-down mode to the fullest,” EDN, July 6, 2000, pg 138.