Embedded systems

sleep instruction

command that puts the device into a low power mode until an interrupt is generated

inherent addressing

nop, return, sleep

no ooperand

absolute addressing

goto and call

allow program to jump to another location (address) in the program

literal (immediate) addressing

addlw, retlw

the destination is always the working register

file direct addressing

instructions that access the data store - addwf, movwf

outcome stored either in the working register or back in the file

bit addressing

bcf, bsf, btfsc, btfss

access the specified bit in a file register

file indirect addressing

internal register, the File Select Register is used to store the data

to trigger, specify the null address (0×00)

to change value stored inside, use the address 0×04

how to load a value into the program counter using PCL and PCLATH

bits <4:0> of the PCLATH will be bits <12:8> of the PC and bits <7:0> come from the PCL.

Write to PCLATH and then write to PCL

how to load a value into the program counter without using PCL

write value to PCLATH if needed

use call instruction, as it takes in a 13 bit literal and the PC is also 13 bits

move value into working register without triggering the flags

swapf 0×21, f followed by swapf 0×21, w as swapf does not affect any status flags

STATUS File 0×03 bits in order

IRP, RP1, PR0, /TO, /PD, Z, DC, C

STATUS IRP<7>

register bank select bit for file indirect addressing

STATUS RP1<6> and RP0<5>

register bank select bits - switch between banks 0, 1, 2 and 3

STATUS /TO<4>

time out

1 = after power-up, clrwdt or sleep instruction

0 = a WDT time out occured

STATUS /PD<3>

power down

1 = after power-up, or by the clrwdt instruction

0 = execution of the sleep instruction

STATUS DC<1>

digit carry / /borrow bit

1 = carry out between the nibbles occured

Little endian for (0×12345678)

leftmost address stores the two LSB (0×78)

Big endian for (0×12345678)

leftmost address stores the two MSB (0×12) - easier to read

stack structure

LIFO - last in first out

stack purpose

hold the subroutine return addresses

stack pointer is post-incremented and pre-decremented

lookup table structure

ORG 0×80

NAME: addwf PCL, f

retlw ?

polling

continously reading a signal

while ( this != that);

INTCON FIle 0×0B bits in order

GIE, PEIE, T0IE, INTE, RABIE, T0IF, INTF, RABIF

1 = enabled or interrupt occured

INTCON GIE<7>

general interrupt enable bit

1 = enable

INTCON PEIE<6>

peripheral interrupt enable bit

UNTCON T0IE<5>

TMR0 overflow interrupt enable bit

INTCON INTE<4>

RA2/INT pin external interrupt enable bit

INTCON RABIE<3>

PORTA/ PORTB change interrupt enable bit

INTCON T0IF<2>

TMR0 overflow interrupt flag bit

INTCON INTF<1>

RA2/INT pin external interrupt flag bit

INTCON RABIF<0>

PORTA/ PORTB change interrupt flab bit

how to enable the hardware interrupt

INTCONbits.GIE = 1;

INTCONbits.INTE = 1;

Program store/ program memory

persistent memory

holds the bit patters that define the operation of the program

RAM data memory/ data store

volatile memory

holds the data being processed by the program

can contain special function registers

EEPROM data memory

programmer readable/ writable persistent data memory

256 bytes of data accessed via special purpose registers EECON1, EECON2, EEDATA and EEADR

Persistent (non-volatile) memory

type of computer memory that can retain stored information even after power is removed

EEPROM and the program store

volatile memory

computer memory that requires power to maintain the stored information

RAM (data store - the file registers)

Configure all of PORT C pins to be digital inputs

set ANSEL and ANSELH to 0 to make them digital

set TRISC to 1 to make them inputs

how would you write a program to divide an 8 bit number by another 8 bit number

subtract the divisor from the dividend repeatedly until a carry (/borrow) is generated and then go back one iteration to get the final solution

program directive

are used to configure various options in relation to the operation of the device, for things such as code protection, enabling watchdog timer and oscillator setup

gives instructions to the assembler on how the program will be assembled or organised

assembly language instruction

line of code that is converted into executable machine code and executed by the CPU

two methods of including assembly language instructions in a C program

either the asm() for a single line of code or #asm to #endasm for multiple lines of code

for a variable to be shared between C and Assembly

ensure both variable definitions refer to the same location in the memory and are accessed using the same symbol name

stack pointer

3 bit register tied to the stack which points to the next available register in the stack

purpose of the TMR0 register

Timer0 module register that can count clock cycles (timer mode) or count external events (counter mode)

set up using the T0IF and T0IE bits in the INTCON register

purpose of the PCL register

stores the lower bytes of the program counter

purpose of the STATUS register

used to store flags that reflect the current state of the CPU and the results of operations

purpose of the FSR register

file select register is used to store the address of the data in file indirect addressing (file 0×04)

purpose of the EEADR register

used to store the address of the EEPROM data memory location that will be accessed in the read or write operation

file 0×10D

purpose of the EEDATA register

used to store the data that is currently being read/ written

file 0×10C

OPTION_REG INTEDG bit

used to select whether the interrupt will be generated on the rising or falling edge of the signal

PIE2 EEIE bit

Peripheral Interrupt Enable Register 2

EE Write Operation Interrupt Enable bit

EEPROM

electrically erasable programmable read only memory

PIR2 EEIF bit

Peripheral interrupt Request Register 2

EE Write Operation Interrupt Flag bit

advantages and disadvantages of using lookup tables

quicker and simpler than performing the calculations + lower CPU usage

takes up more memory space, only works for predefined values

char

8 bits (1 byte)

short

16 bits (2 bytes)

int

16 bits (2 bytes)

long

32 bits (4 bytes)

float

24 bits (3 bytes)

double

24 or 32 bits (3 or 4 bytes)

how to disable the oscillator and place the PIC in a low power standby mode

sleep instruction

special function register

used to control and monitor the internal hardware features of the device

INTCON, STATUS, TRISX, TMR0

reset vector

address 0×000 that the program jumps to on reset

interrupt vector

address 0×004 that the program jumps to whenever an interrupt is generated

Arithmetic and logic unit

carries out an arithmetic or logic operation as commanded by the mode code which is extracted from the instruction by the instruction decoder

Program store

each location has an address and holds one 14 bit long instruction

Data store

each file holds one byte of data. the file address is generated by the execute unit via the File Address Register and the contents of the addressed file are either read into the File Data Register or written to from it

File Address Register

when the CPU wants to access a file in the data store the address of the file goes into the FAR via the FA bus

File Data Register

bidirectional register which holds the contents of an addressed file of the CPU is executing a read cycle or that the CPU wants to send out to an existing memory location

why the goto instruction should never be used to call a subroutine

the goto instruction is unidirectional and it does not provide the program with a return address to be placed on the stack that the program can jump to when it finishes executing the subroutine