Microcontrollers: PIC16

Microcontrollers are basically scaled-down computers. They have an Arithmetic Logic Unit, program memory, memory stack, registers and an all-out assembly instruction set. Instead of a monitor and keyboard, they have many pins and ports that can be configured various ways. Many companies produce microcontrollers. I went with the PIC18F4550 Microcontroller from Microchip. But, let's start with the PIC16F84A.

Here is a block diagram of the PIC16F84A microcontroller chip. (click to enlarge)

You will need a few items to begin working with microcontrollers. I purchased the PICSTART Plus Part Number: DV003001 kit from Microchip. This kit includes the following:

A sample PIC16F84 microcontroller chip: My kit actually came with a PIC16F84A chip. This is a 18 pin, 8-bit chip. The PIC16F84A data sheet is only 88 pages. So this is easier than the 18F chip. But the 16F chips do not support USB. The programmer unit: This is the device that will write the program to the chip. This will interface with a computer's serial port. On newer computers, you may need an additional USB to Serial Converter. I am able to use this programmer for the 18 pin PIC16F and the 40 pin PIC18F chips. Compiler, Assembler and IDE. The kit comes with a copy of the MPLAB IDE. This provides a development environment for many different chips and supports C and Assembler programming languages. The IDE contains a simulator and a debugger.

While you wait for the kit to be delivered, I would recommend reading the PIC16F84A data sheet. You will need to review the pin configuration on page 3. Make special note of the following:

Pin Name Pin Number Description MCLR 4 Master Clear Reset Vss 5 Ground Vdd 14 Positive OSC2 15 Oscillator crystal output OSC1 16 Oscillator crystal input

You will need a breadboard, a IC Extraction Tool, a 5 volt regulator (7805), some LEDs and resistors. I used low power 15mA 3volt LEDs. Doing the math, you find R = (5volt - 3volt)/(15mA) = 133 kOhm. So, I used 150 kOhm resistors with these LEDs. You can find these at a local RadioShack. You will need a power source. I use a 12 Volt battery jump box. Note: Use a fuse between the power source and your circuit. You don't want a 12V 300Amp short circuit!

Here is my schematic for a test circuit with the PIC16F84A:

Here is a photo of the actual circuit. This image has balloon hot-spots.

Here is the assembly code. The goal of this program is to flash the numbers 0 to 15 in binary on the LEDs. Here is a copy of the MPLAB IDE project files.

list p=16F84A #include <p16F84A.inc> ; processor specific variable definitions radix hex __CONFIG _CP_OFF & _WDT_OFF & _PWRTE_ON & _XT_OSC udata Delay1 res 1 ; reserve a byte for a delay counter Delay2 res 1 ; reserve another byte for a delay counter led_counter res 1 ; reserve one byte for our led counter org 0x005 start movlw b'00001111' ; load W with b'00001111' ; we will reserve RB7,RB6,RB5,RB4 for output tris PORTB ; enable the selected pins for output clrf PORTB ; clear PORT B movlw b'00010000' ; move b'00001111' to register w movwf led_counter ; move register w to led_counter my_loop call change_leds ; changed the leds call pause ; wait a few cycles goto my_loop ; repeat ; ------------------------------------ ; pause ; ------------------------------------ pause my_delay: DECFSZ Delay1,1 ;Decrement Delay1 by 1, skip next instruction if Delay1 is 0 GOTO my_delay DECFSZ Delay2,1 GOTO my_delay return ; ------------------------------------ ; change_lights ; ------------------------------------ change_leds decf led_counter,1 ; led_counter = led_counter - 1 swapf led_counter,0 ; we need to shift the value of led_counter ; so that the lower half of the byte ; displays on pins 4-7 ; This new value will be in register w movwf PORTB ; move w to PORTB movfw led_counter bz reset_counter ; if led_counter == 0, then jump to 'reset_counter' return reset_counter ; reset led_counter to b'00010000' movlw b'00010000' ; move b'00001111' to register w movwf led_counter ; move register w to led_counter return end