1. AN557 Microchip Four Channel Digital Voltmeter with Display and Keyboard|
The PIC16C71's 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 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 are used to sink the digit current (Figure 1). Another alternative is to use ULN2003 open collector sink current drivers, which are available in 16-pin DIP or very small SO-16 packages. Each transistor on the ULN2003 can sink a maximum of 500 mA and the base drive can be directly driven from the PORTA pins. 30 pages.