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2.1 Clock generator - oscillator

Oscillator circuit is used for providing a microcontroller with a clock. Clock is needed so that microcontroller could execute a program or program instructions.

Types of oscillators

PIC16F84 can work with four different configurations of an oscillator. Since configurations with crystal oscillator and resistor-capacitor (RC) are the ones that are used most frequently, these are the only ones we will mention here. Microcontroller type with a crystal oscillator has in its designation XT, and a microcontroller with resistor-capacitor pair has a designation RC. This is important because you need to mention the type of oscillator when buying a microcontroller.

XT Oscillator

Crystal oscillator is kept in metal housing with two pins where you have written down the frequency at which crystal oscillates. One ceramic capacitor of 30pF whose other end is connected to the ground needs to be connected with each pin.

Oscillator and capacitors can be packed in joint case with three pins. Such element is called ceramic resonator and is represented in charts like the one below. Center pins of the element is the ground, while end pins are connected with OSC1 and OSC2 pins on the microcontroller. When designing a device, the rule is to place an oscillator nearer a microcontroller, so as to avoid any interference on lines on which microcontroller is receiving a clock.

Chapter 2 - Microcontroller PIC16F84

To prevent such inaccurate clock from influencing microcontroller's performance, we need to keep the microcontroller in reset state during stabilization of oscillator's clock. Above diagram shows a typical shape of a signal which microcontroller gets from the quartz oscillator following a supply.

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2.2 Reset

Reset is used for putting the microcontroller into a 'known' condition. That practically means that microcontroller can behave rather inaccurately under certain undesirable conditions. In order to continue its proper functioning it has to be reset, meaning all registers would be placed in a starting position. Reset is not only used when microcontroller doesn't behave the way we want it to, but can also be used when trying out a device as an interrupt in program execution, or to get a microcontroller ready when reading in a program.

In order to prevent from bringing a logical zero to MCLR pin accidentally (line above it means that reset is activated by a logical zero), MCLR has to be connected via resistor to the positive supply pole. Resistor should be between 5 and 10K. This kind of resistor whose function is to keep a certain line on a logical one as a preventive, is called a pull up.

Microcontroller PIC16F84 knows several sources of resets:

a)Reset during power on, POR (Power-On Reset)

b)Reset during regular work by bringing logical zero to MCLR microcontroller's pin.

c)Reset during SLEEP regime

d)Reset at watchdog timer (WDT) overflow

e)Reset during at WDT overflow during SLEEP work regime.

The most important reset sources are a) and b). The first one occurs each time a power supply is brought to the microcontroller and serves to bring all registers to a starting position initial state. The second one is a product of purposeful bringing in of a logical zero to MCLR pin during normal operation of the microcontroller. This second one is often used in program development.

During a reset, RAM memory locations are not being reset. They are unknown during a power up and are not changed at any reset. Unlike these, SFR registers are reset to a starting position initial state. One of the most important effects of a reset is setting a program counter (PC) to zero (0000h) , which enables the program to start executing from the first written instruction.

Chapter 2 - Microcontroller PIC16F84

Reset at supply voltage drop below the permissible (Brown-out Reset)

Impulse for resetting during voltage voltage-up is generated by microcontroller itself when it detects an increase in supply Vdd (in a range from 1.2V to 1.8V). That impulse lasts 72ms which is enough time for an oscillator to get stabilized. These 72ms are provided by an internal PWRT timer which has its own RC oscillator. Microcontroller is in a reset mode as long as PWRT is active. However, as device is working, problem arises when supply doesn't drop to zero but falls below the limit that guarantees microcontroller's proper functioning. This is a likely case in practice, especially in industrial environment where disturbances and instability of supply are an everyday occurrence. To solve this problem we need to make sure that microcontroller is in a reset state each time supply falls below the approved limit.

If, according to electrical specification, internal reset circuit of a microcontroller can not satisfy the needs, special electronic components can be used which are capable of generating the desired reset signal. Beside this function, they can also function in watching over supply voltage. If voltage drops below specified level, a logical zero would appear on MCLR pin which holds the microcontroller in reset state until voltage is not within limits that guarantee correct functioning.

Chapter 2 - Microcontroller PIC16F84

Chapter 2 - Microcontroller PIC16F84

or right within a register) and logic operations. Moving data inside a register is also known as 'shifting'. PIC16F84 contains an 8-bit arithmetic logic unit and 8-bit work registers.

In instructions with two operands, ordinarily one operand is in work register (W register), and the other is one of the registers or a constant. By operand we mean the contents on which some operation is being done, and a register is any one of the GPR or SFR registers. GPR is an abreviation for 'General Purposes Registers', and SFR for 'Special Function Registers'. In instructions with one operand, an operand is either W register or one of the registers. As an addition in doing operations in arithmetic and logic, ALU controls status bits (bits found in STATUS register). Execution of some instructions affects status bits, which depends on the result itself. Depending on which instruction is being executed, ALU can affect values of Carry (C), Digit Carry (DC), and Zero (Z) bits in STATUS register.

Chapter 2 - Microcontroller PIC16F84

bit 0 C (Carry) Transfer

Bit that is affected by operations of addition, subtraction and shifting. 1= transfer occured from the highest resulting bit

0=transfer did not occur

C bit is affected by ADDWF, ADDLW, SUBLW, SUBWF instructions.

bit 1 DC (Digit Carry) DC Transfer

Bit affected by operations of addition, subtraction and shifting. Unlike C bit, this bit represents transfer from the fourth resulting place. It is set by addition when occurs carry from bit3 to bit4, or by subtraction when occurs borrow from bit4 to bit3, or by shifting in both direction. 1=transfer occured on the fourth bit according to the order of the result

0=transfer did not occur

DC bit is affected by ADDWF, ADDLW, SUBLW, SUBWF instructions.

bit 2 Z (Zero bit) Indication of a zero result

This bit is set when the result of an executed arithmetic or logic operation is zero. 1=result equals zero

0=result does not equal zero

bit 3 PD (Power-down bit)

Bit which is set whenever power supply is brought to a microcontroller as it starts running, after each regular reset and after execution of instruction CLRWDT. Instruction SLEEP resets it when microcontroller falls into low consumption/usage regime. Its repeated setting is possible via reset or by turning the supply on, or off . Setting can be triggered also by a signal on RB0/INT pin, change on RB port, completion of writing in internal DATA EEPROM, and by a watchdog, too. 1=after supply has been turned on

0= executing SLEEP instruction

bit 4 TO Time-out ; Watchdog overflow.

Bit is set after turning on the supply and execution of CLRWDT and SLEEP instructions. Bit is reset when watchdog gets to the end signaling that something is not right.

1=overflow did not occur 0=overflow did occur

bit6:5 RP1:RP0 (Register Bank Select bits)

These two bits are upper part of the address for direct addressing. Since instructions which address the memory directly have only seven bits, they need one more bit in order to address all 256 bytes which is how many bytes PIC16F84 has. RP1 bit is not used, but is left for some future expansions of this microcontroller.

01=first bank 00=zero bank

Chapter 2 - Microcontroller PIC16F84

bit 7 IRP (Register Bank Select bit)

Bit whose role is to be an eighth bit for indirect addressing of internal RAM. 1=bank 2 and 3

0=bank 0 and 1 (from 00h to FFh)

STATUS register contains arithmetic status ALU (C, DC, Z), RESET status (TO, PD) and bits for selecting of memory bank (IRP, RP1, RP0). Considering that selection of memory bank is controlled through this register, it has to be present in each bank. Memory bank will be discussed in more detail in Memory organization chapter. STATUS register can be a destination for any instruction, with any other register. If STATUS register is a destination for instructions which affect Z, DC or C bits, then writing to these three bits is not possible.

OPTION register

bit 0:2 PS0, PS1, PS2 (Prescaler Rate Select bit)

These three bits define prescaler rate select bit. What a prescaler is and how these bits can affect the work of a microcontroller will be explained in section on TMR0.

bit 3 PSA (Prescaler Assignment bit)

Bit which assigns prescaler between TMR0 and watchdog. 1=prescaler is assigned to watchdog

0=prescaler is assigned to a free-run timer TMR0

bit 4 T0SE (TMR0 Source Edge Select bit)

If it is allowed to trigger TMR0 by impulses from the pin RA4/T0CKI, this bit determines whether this will be to the falling or rising edge of a signal.

1=falling edge 0=rising edge

Chapter 2 - Microcontroller PIC16F84

bit 5 TOCS (TMR0 Clock Source Select bit)

This pin enables free-run timer to increment its state either from internal oscillator on every ¼ of oscillator clock, or through external impulses on RA4/T0CKI pin.

1=external impulses 0=1/4 internal clock

bit 6 INTEDG (Interrupt Edge Select bit)

If interrupt is enabled possible this bit will determine the edge at which an interrupt will be activated on pin RB0/INT.

1=rising edge 0=falling edge

bit 7 RBPU (PORTB Pull-up Enable bit)

This bit turns on and off internal 'pull-up' resistors on port B. 1= "pull-up" resistors turned off

0= "pull-up" resistors turned on

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