Components of a Microcontroller

The microcontroller (MCU) has on-chip program memory (Figure 1 or Figure 2) for the firmware, or coded instructions, to run a program (Figure 3 or Figure 4). A Program Counter (PC) is used to address program memory, including Reset and interrupt addresses. A hardware stack is used with call and return instructions in code, so it works with, but is not part of, program memory. Device data sheets describe the details of program memory operation, vectors and the stack.

Figure 1. PIC® MCU Data Sheet - Program Memory and Stack

Figure 2. AVR® MCU Data Sheet - Program Memory

Figure 3. PIC® MCU Data Sheet – Instruction Set (Excerpt)

Figure 4. AVR® MCU Instruction Set (Excerpt)

The microcontroller also has data, or File Register, memory. This memory consists of Special Function Registers (SFRs) and General Purpose Registers (GPRs). SFRs are registers used by the CPU and peripheral functions for controlling the desired operation of the device. GPRs are for storage of variables that the program will need for computation or temporary storage. Some microcontrollers have additional data EEPROM memory. As with program memory, device data sheets describe the details of data memory use and operation.

Table 1. PIC® MCU Data Sheet – File Registers Example
Bank 0	File Address	Bank 1	File Address	Bank 2	File Address	Bank 3	File Address
Indirect addr. ⁽¹⁾	00h	Indirect addr. ⁽¹⁾	80h	Indirect addr. ⁽¹⁾	100h	Indirect addr. ⁽¹⁾	180h
TMR0	01h	OPTION_REG	81h	TMR0	101h	OPTION_REG	181h
PCL	02h	PCL	82h	PCL	102h	PCL	182h
STATUS	03h	STATUS	83h	STATUS	103h	STATUS	183h
FSR	04h	FSR	84h	FSR	104h	FSR	184h
PORTA	05h	TRISA	85h	PORTA	105h	TRISA	185h
PORTB	06h	TRISB	86h	PORTB	106h	TRISB	186h
PORTC	07h	TRISC	87h	PORTC	107h	TRISC	187h
	08h		88h		108h		188h
	09h		89h		109h		189h
PCLATH	0Ah	PCLATH	8Ah	PCLATH	10Ah	PCLATH	18Ah
INTCON	0Bh	INTCON	8Bh	INTCON	10Bh	INTCON	18Bh
PIR1	0Ch	PIE1	8Ch	EEDAT	10Ch	EECON1	18Ch
PIR2	0Dh	PIE2	8Dh	EEADR	10Dh	EECON2⁽¹⁾	18Dh
TMR1L	0Eh	PCON	8Eh	EEDATH	10Eh		18Eh
TMR1H	0Fh	OSCCON	8Fh	EEADRH	10Fh		18Fh
T1CON	10h	OSCTUNE	90h		110h		190h
TMR2	11h		91h		111h		191h
T2CON	12h	PR2	92h		112h		192h
SSPBUF	13h	SSPADD⁽²⁾	93h		113h		193h
SSPCON	14h	SSPSTAT	94h		114h		194h
CCPR1L	15h	WPUA	95h	WPUB	115h		195h
CCPR1H	16h	IOCA	96h	IOCB	116h		196h
CCP1CON	17h	WDTCON	97h		117h		197h
RCSTA	18h	TXSTA	98h	VRCON	118h		198h
TXREG	19h	SPBRG	99h	CM1CON0	119h		199h
RCREG	1Ah	SPBRGH	9Ah	CM2CON0	11Ah		19Ah
	1Bh	BAUDCTL	9Bh	CM2CON1	11Bh		19Bh
PWM1CON	1Ch		9Ch		11Ch		19Ch
ECCPAS	1Dh		9Dh		11Dh	PSTRCON	19Dh
ADRESH	1Eh	ADRESL	9Eh	ANSEL	11Eh	SRCON	19Eh
ADCON0	1Fh	ADCON1	9Fh	ANSELH	11Fh		19Fh
General  Purpose  Register 96 Bytes	20h	General  Purpose  Register 80 Bytes	A0h	General  Purpose  Register 80 Bytes	120h		1A0h
		General  Purpose  Register 80 Bytes	EFh	General  Purpose  Register 80 Bytes	16Fh
		accesses 70h-7Fh	F0h	accesses 70h-7Fh	170h	accesses 70h-7Fh	1F0h
	7Fh	accesses 70h-7Fh	FFh	accesses 70h-7Fh	17Fh	accesses 70h-7Fh	1FFh
Empty table cells: Unimplemented data memory locations, read as ‘0’. Note 1: Not a physical register. Note 2: Address 93h also accesses the SSP Mask (SSPMSK) register under certain conditions.

Figure 5. AVR® MCU Data Sheet – SRAM Data Memory and Stack

In addition to memory, the microcontroller has a number of peripheral device circuits on the same chip. Some peripheral devices are called input/output (I/O) ports. I/O ports are pins on the microcontroller that can be used as outputs and driven high or low to send signals, blink lights, drive speakers – just about anything that can be sent through a wire. Often these pins are bidirectional and can also be configured as inputs, allowing the program to respond to an external switch, a sensor, or to communicate with some external device.

Figure 6. PIC® MCU Data Sheet – Block Diagram (Excerpt)

Figure 7. AVR® MCU Data Sheet – Block Diagram (Excerpt)

To design such a system, choose which peripherals are necessary for the application.

The following is a list of common peripherals:

Analog-to-Digital Converters (ADCs) allow microcontrollers to connect to sensors and receive changing voltage levels.
Serial communication peripherals that allow streaming communications over a few wires to another microcontroller, to a local network, or to the Internet.
Peripherals on the PIC MCU called “timers” accurately measure signal events and generate and capture communications signals, produce precise waveforms, even automatically reset the microcontroller if it gets “hung” or lost due to a power glitch or hardware malfunction.
Other peripherals detect when the external power is dipping to dangerous levels, so that the microcontroller can store critical information and safely shut down before power is completely lost.

The peripherals, and the amount of memory an application needs to run a program, largely determine which PIC MCU to use. Other factors could include the power consumed by the microcontroller and its “form factor,” i.e., the size and characteristics of the physical package that must reside on the target design.

Figure 8. PIC® MCU Device Package Example

Figure 9. Example AVR® MCU Device Package

A microcontroller becomes active when power is applied and an oscillator begins generating a clocking signal. Depending on the microcontroller, there may be several internal and external oscillator operational modes.

Figure 10. PIC® MCU Data Sheet – Timing Diagram (Excerpt)

Figure 11. AVR® MCU Data Sheet – Timing (Excerpt)