Electronic Design

Keyboard scanner provides system supervision

The circuit shown scans a numeric keypad while also generating 16-ms timing interrupts, low-line resets, and power-fail warnings for a larger system (see the figure). The core of the circuit is a versatile PIC16C54 microcontroller augmented by a microprocessor supervisor (IC3).

In place of the microcontroller shown in the figure, most systems typically employ dedicated logic—a keyboard-interface IC such as the 74C922, a time-base generator such as the 74C4060, a 32-kHz crystal, and a microprocessor supervisor. The small and inexpensive PIC, however, offers design flexibility in exchange for some simple code development (see the listing part a and part b).

The microprocessor supervisor generates activehigh and active-low resets in response to a low VCC voltage. It comes in 5-V and 3.3-V versions; the one shown is a 5-V type with a trip threshold of 4.4 V. It also monitors the battery via the power-fail input (PFI), with a trip threshold of 6.4 V set by R1 and R2. Battery status is read by the PIC at IC3’s !PFO output, and encoded as the MSB (RB7) of an 8-bit data word returned to the main processor every 16 ms.

Other data bits in this byte pertain to the keypad—a 4-by-4 matrix-type such as the Grayhill 84S-BC series. The PIC scans the keypad for contact closures by issuing consecutive “1”s on the low nibble of the bidirectional RB port (RB0-RB3) and watching for a tell-tale “1” on the high nibble. It then converts this row-column information to a 4-bit code, issues it on RB1-RB4 (along with the rest of the byte), enables the latch (IC1), and issues an interrupt. If no key was pressed, the key code is all “0”s on RB0-RB4 (otherwise RB0 is a “1”).

The duration of each contact also is measured by the PIC. A minimum 32-ms interval (two scans or ticks) verifies a valid closure and debounces the contacts. Closure duration is encoded as 0 to 1 tick, 2 to 10 ticks (<0.16 seconds), 10 to 20 ticks (0.16 to 0.32 seconds), or >20 ticks (>0.32 seconds), and returned to the system as data bits RB5 and RB6. This data enables the main processor to provide an autorepeat function for the keypad.

Finally, the crystal-accurate 16-ms interrupts provide a time base for the processor that eliminates the need for a separate time-base generator and crystal. Other advantages (over the discrete CMOS MSI approach) include: flexibility—the software allows for easy changes in the keyboard code, auto-repeat time, debounce delay, interrupt rate, etc.. Board space⇓the PIC is available in a surface-mount package and IC3 comes in a tiny shrink SO-8 “µMax” package. Cost and reliability—the PIC uses a standard AT crystal instead of the fragile 32-kHz-type otherwise required.

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