Microcontroller based applied digital control (D. Ibrahim, 2006)

64 THE PIC MICROCONTROLLER

Figure 3.8 Using a crystal

Table 3.7 Capacitor values for crystal operation

Figure 3.9 Using a resonator

3.3.1 PIC16F84 Microcontroller

The PIC16F84 is one of the most popular PIC microcontrollers used in many commercial, industrial and hobby applications. This is an 18-pin device which can operate at up to 20 MHz clock speed. It offers 1024 × 14 flash program memory, 68 bytes of RAM data memory, 64 bytes of EEPROM nonvolatile data memory, 8-bit timer with pre-scaler, watchdog timer, 13 I/O pins, external and internal interrupt sources, and large current sink and source capability.

Figure 3.13 shows the pin configuration of the PIC16F84. The functions of various pins are as follows:

Note that some pin names have a bar on them – for example, MCLR in Figure 3.13. This means that the pin will be active when the applied signal is at logic low (logic 0 in this case).

The PIC16F84 provides four external or internal interrupt sources:

external interrupt on INT pin;

timer overflow interrupt;

state change interrupt on the four higher bits of port B (RB4–RB7);

EEPROM memory data write complete interrupt.

The data RAM is also known as the register file map (RFM) and consists of 80 bytes. As shown in Figure 3.14, the RFM is divided into two parts: the special function registers (SFR), and the general purpose registers (GPR). The RFM is organized as two banks (more complex PIC microcontrollers may have more banks): bank 0 and bank 1. The bank of a register must

Figure 3.13 PIC16F84 pin configuration

68 THE PIC MICROCONTROLLER

Figure 3.14 Register file map of PIC16F84

be selected before a register in a bank can be read or written to. Some of the registers are common to both banks.

The SFR are a collection of registers used by the CPU to control the internal operations and the peripherals – setting the I/O direction of a register, sending data to an I/O port, loading the timer register, etc. The SFR used while programming the microcontroller using a high-level language are described in the following sections.

3.3.1.1 OPTION REG Register

The OPTION REG register is a readable and writable register at address 0×81 (hexadecimal) of the RFM. This register controls the timer pre-scaler, timer clock edge selection, timer clock source, external interrupt edge selection, and port B pull-up resistors. OPTION REG bit definitions are given in Figure 3.15. For example, to configure the external interrupt INT pin so that external interrupts are accepted on the falling edge of the INT input, the following bit pattern should be loaded into the OPTION REG:

X0XXXXXX

where X is a don’t-care bit and can be a 0 or a 1.

3.3.1.2 INTCON Register

This is the interrupt control register at addresses 0×0B and 0×8B of the RFM. The bit definitions of this register are shown in Figure 3.16. INTCON is used to enable/disable the various interrupt sources and interrupt flags. For an interrupt to be accepted by the CPU, the

Bit 7: PORTB pull-up control

1: PORTB pull-ups disabled 0: PORTB pull-ups enabled

Bit 6: INT external interrupt edge detect

1: Interrupt on rising edge of INT input 0: Interrupt on falling edge of INT input

Bit 5: TMR0 timer clock source 1: T0CK1 external pulse 0: internal clock

Bit 4: TMR0 source edge select

1: Increment on HIGH to LOW of T0CK1 0: Increment on LOW to HIGH of T0CK1

Bit 3: Pre-scaler assignment

1: Pre-scaler assigned to watchdog timer 0: Pre-scaler assigned to TMR0

Bit 2-0: Pre-scaler rate

1001:32

1011:64

1101:128

1111:256

Figure 3.15 OPTION REG bit definitions

Bit 7: Global interrupt control

1: Enable all unmasked interrupts 0: Disable all interrupts

Bit 6: EEPROM write complete interrupt

1: Enable EEPROM write complete interrupt 0: Disable EEPROM write complete interrupt

Bit 5: TMR0 overflow interrupt 1: Enable TMR0 interrupt

0: Disable TMR0 interrupt

Bit 4: INT external interrupt control

1: Enable INT External interrupt 0: Disable INT External Interrupt

Bit 3: RB4–RB7 port change interrupt control

1: Enable RB4–RB7 port change interrupt 0: Disable RB4–RB7 port change interrupt

Bit 1: INT interrupt flag

1: INT interrupt occurred

0: INT interrupt did not occur

Bit 0: RB4–RB7 port change interrupt flag

1: One or more of RB4–RB7 pins changed state 0: None of RB4–RB7 pins changed state

Figure 3.16 INTCON bit definitions

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following conditions must be met:

The global interrupt flag in INTCON must be enabled (GIE = 1).

The interrupt flag of the interrupting source in INTCON must be enabled (e.g. INTE = 1 to enable INT external interrupts).

Interrupt must physically occur (e.g. INT pin is raised to logic 1 if INTEDG was previously set to logic 1).

After an interrupt is detected the program jumps to the interrupt service routine which is at address 4 of the program memory. At this point further interrupts are disabled and the interrupt flag of the interrupt source (e.g. bit INTF of INTCON for external interrupts) must be cleared for a new interrupt to be accepted from the interrupting source.

3.3.1.3 TRISA and Port A Registers

Port A is a 5-bit-wide port with pins RA0–RA4, and at address 5 of the RFM. Four low-order bits (RA0–RA3) have CMOS output drivers with 25 mA current sink and source capabilities. RA4 is an open-drain port and a suitable pull-up resistor must be connected when used as an output port. Port A pins are bidirectional and the direction of a pin is determined by the settings of register TRISA. Setting a bit in TRISA makes the corresponding port A pin an input. Similarly, clearing a bit in TRISA makes the corresponding port A pin an output. For example, to make bits 0, 1 and 2 of port A input and the other bits output, we have to load TRISA register with:

00000111

3.3.1.4 TRISB and Port B Registers

Port B is a 8-bit-wide port with pins RB0–RB7, and at address 6 of the RFM. The pins have CMOS output drivers with 25 mA current sink and source capabilities. Pin RB0 can be used as an external interrupt pin. Similarly, pins RB4–RB7 can be used to generate an interrupt when the state of any of these pins changes. Port B pins are bidirectional, and the direction of a pin is determined by the settings of register TRISB. Setting a bit in TRISB makes the corresponding port B pin an input. Similarly, clearing a bit in TRISB makes the corresponding port B pin an output. For example, to make bits 0, 2 and 4 of port B input and the other bits output, we have to load TRISB register with:

00010101

3.3.1.5 TMR0 Register

The PIC16F84 provides an 8-bit timer, called TMR0, which can be used either as a timer or a counter. The structure of this timer is shown in Figure 3.17. When used as a counter, the register increments each time a clock pulse is applied to external pin T0CK1 of the microcontroller.

watchdog timer;

14 interrupt sources;

capture, compare and PWM modules;

on-chip USART;

25 mA current source and sink capability.

Figure 3.18 shows the pin configuration of the PIC16F877. I/O ports are accessed as in the PIC16F84 where each port has a direction register (TRIS) which determines the mode of the I/O pins. One of the nice features of the PIC16F877 is that it contains a multiplexed eightchannel A/D converter with 10-bit resolution. A/D conversion is important in microcontroller based control applications, and the operation of this module is described in more detail below.

3.3.2.1 A/D Converter

The eight A/D converter inputs are named AN0–AN7 and are shared with PORTA and PORTE digital inputs as shown in Figure 3.19. There is only one A/D converter and the analog inputs are multiplexed where only one analog input data is converted to digital at any time. Analog