This electronic circuit project is a simple four channel digital voltmeter with display and keyboard .
This four channel digital voltmeter is based on PIC16C71 manufactured by Microchip Technology . The PIC16C71 deviceβs I/O ports have an improved sink/source specification. Each I/O pin can sink up to 25 mA and source 20 mA. In addition, total PORTB source current is 100 mA and sink current is 150 mA.
PORTA is rated for a 50 mA source current and 80 mA sink current. This makes the PIC16C71 ideal for driving 7-segment LEDs. Since the total number of I/O pins is limited to 13, the 8-bit PORTB is used to drive the 4 LEDs, while external sink transistors, or MOSFETs.
The multiplexing is achieved by turning on each LED for a 5 ms duration every 20 ms. This gives an update rate of 50 Hz, which is quite acceptable to the human eye as a steady display. The 5 ms time base is generated by dividing the 4.096 MHz oscillator clock. The internal prescaler is configured to be a divide by 32 and assigned to Timer0. TMR0 is preloaded with a value = 96. TMR0 will increment to FFh and then roll over to 00h after a period = (256 β 96) β’ (32 β’ 4/4096000) = 5 ms.
When TMR0 rolls over, the T0IF flag bit is set, and because bits T0IE and GIE are enabled, an interrupt is generated.
A 4Γ4 keypad can very easily be interfaced to the PIC16C71 deviceβs PORTB .
The internal pull-ups have a value of 20k at 5V (typical). In order to sense a low level at the input, the switch is βconnectedβ to ground through a 2.2 kΞ© resistor. A key hit normally lasts anywhere from 50 ms to as long as a person holds the key down. In order not to miss any key hits, the keypad is sampled every 20 ms (just after the update of the MSD).
The software implements a simple timer which increments at a 1-second rate. Every second, the 4 nibbles (two 8-bit registers, MsdTime and LsdTime) are incremented in a BCD format.
The analog channels are connected through individual potentiometers to their respective analog inputs and are sampled every 20 ms in a round robin fashion. The sampling rate can be increased to as fast as once every 5 ms if required. The keypad sampling need not be any faster than once every 20 ms.
This project and source code was designed by Stan DβSouza from Microchip Technology.
For more detail: PIC16c71 four channel digital voltmeter
