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

3.2 MINIMUM PIC CONFIGURATION

The minimum PIC configuration depends on the type of microcontroller used. Normally, the operation of a PIC microcontroller requires a power supply, reset circuit and oscillator.

The power supply is usually +5 V and, as shown in Figure 3.3, can be obtained from the mains supply by using a step-down transformer, a rectifier circuit and a power regulator chip, such as the LM78L05.

Although PIC microcontrollers have built-in power-on reset circuits, it is useful in many applications to have external reset circuits. When the microcontroller is reset, all of its special function registers are put into a known state and execution of the user program starts from address 0 of the program memory.

As shown in Figure 3.4, reset is normally achieved by connecting a 4.7 K pull-up resistor from the master clear (MCLR) input to the supply voltage. Sometimes the voltage rises too slowly and the simple reset circuit may not work. In this case, the circuit shown in Figure 3.5 is recommended.

In many applications it may be required to reset the microcontroller by pressing an external button. The circuit given in Figure 3.6 enables the microcontroller to reset when the button is pressed.

PIC microcontrollers have built-in clock oscillator circuits. Additional components are needed to enable such clock oscillator circuits to function; some PIC microcontrollers have these built in, while others require external components. The internal oscillator can be operated in one of six modes:

external oscillator;

LP – low-power crystal;

XT – low-speed crystal/resonator;

Figure 3.3 A simple microcontroller power source

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Figure 3.4 Simple reset circuit

Figure 3.5 Recommended reset circuit

Figure 3.6 Push-button reset circuit

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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

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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