PIC18F Architecture

The PIC18F family is an 8-bit RISC microcontroller from Microchip with Harvard architecture (separate program and data buses), so instructions and data are fetched at the same time.

Main features

Block	Description
CPU	8-bit, 16-bit instruction width, about 75 instructions, hardware multiplier 8x8
Program memory	Flash, 16 KB to 128 KB
Data memory	SRAM up to 4 KB, banked, plus special function registers
EEPROM	Non-volatile storage for settings
I/O ports	PORTA to PORTE with TRIS direction registers
Peripherals	10-bit ADC, comparators, CCP/PWM, UART, SPI, I2C
Interrupts	Priority-based, high and low levels
Clock	Internal or external oscillator with PLL

Timers and counters

A timer counts internal clock pulses ($f_{osc}/4$), so it measures time. A counter counts external pulses on a pin, so it counts events.

Timer	Size	Typical use
Timer0	8 or 16 bit	Delays, event counting
Timer1	16 bit	Real-time clock, capture
Timer2	8 bit	PWM time base
Timer3	16 bit	Capture/compare

Delay: $T = (65536 - preload) \times prescaler \times 4/f_{osc}$.

Applications: periodic ADC sampling, PWM motor control, pulse width and frequency measurement, timeouts.

Exam tip

Draw the block diagram, name Harvard architecture and give the timer vs counter difference.