The PIC Microcontrollers

The PIC microcontroller was designed by General Instruments The PIC microcontroller is a complete computer contained within one chip It contains the CPU, memory and IO devices all in one chip The microcontroller contains many IO ports such as PORTA, B, C, D, E It contains also timers, analog to digital converter, USART This make this device an ideal solution for small project1

MEMORY ORGANIZATION

There are three memory blocks in each of the PIC16F87A devices1. 2. 3.

The program memory The data memory Stack memory

The program memory and data memory have separate buses which allows concurrent access

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Program Memory Organization

The PIC microcontroller contains many internal registers to control its operation One of these registers is the program counter The program counter is 13-bits wide in the 16F877A microcontrollers This limits the program memory space to 213=8k words14 bits of flash program memory When the microprocessor is first powered up it starts execution at memory location 0000H When an interrupt occurs the microprocessor goes to memory location 0004H 3

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Data Memory Organization

The data memory is partitioned into multiple banks which contains 1. The General Purpose Registers 2. The Special Function Registers

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Data Memory Organization

A given bank is selected by bits RP1 (Status) and RP0 (Status) according to the table shown below These two bits are located in the status register which is equivalent to the flag register found in the 8086

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SPECIAL FUNCTION REGISTERS

The Special Function Registers are registers used by the CPU and peripheral modules for controlling the desired operation of the device These registers are shown in the attached sheets The most important registers will be discussed next

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Status Register

The Status register contains the flag bits, reset status and the bank select bits for data memory

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OPTION_REG Register

This register contains various control bits to configure1. the TMR0 prescaler/WDT postscaler (single

assignable 2. register known also as the prescaler), the external 3. INT interrupt, TMR0 and the weak pull-ups on PORTB

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INTCON Register

This register is responsible controlling hardware interrupts The GIE (bit 7) is responsible for enable or disable of all interrupts in the system If GIE is 1 then all interrupts will be enabled otherwise all interrupts will be disabled A complete description of these interrupts will be provided in the next slide

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PIE1 Register

The PIE1 register contains the individual enable/disable bits for the peripheral interrupts Peripherals are1.2. 3. 4. 5. 6.

the analog to digital converter USART receiver USART transmitter Synchronous Serial Port Timer1 Timer2

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PIR1 Register

When an interrupt is generated, it is important to know which one of the above mentioned peripherals generates the interrupt This can be done by inspecting the state of the PIR1 register which contains several flag bits corresponds to each peripheral If an interrupt occurs, then the program can test the flags located in this register to determine which device generated the interrupt A complete explanation for this register is provided in the next slide19

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PCON Register

The Power Control (PCON) register contains flag bits to allow differentiation between a Power-on Reset (POR), a Brown-out Reset (BOR), a Watchdog Reset (WDT) and an external MCLR Reset

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