PIC micro14 Architecture Video... some important ideas (but not an exhaustive list!!)


Some dimensions:
    8 bit data bus
    14 bit instruction bus
    13 bit PC
    8 level stack

Addressing Modes:

1) Direct
        7 bits of the instruction specify a register. 7 bits --> 128 different 8-bit registers. But PIC 16f627/628 can address more registers. How ? Status register contains two bits (RP0 and RP1) that can specify 4 different register banks. So maximum theoretical RAM space is 4 x 128 = 512 .

2) Indirect
        Use the FSR register for indirect addressing. FSR register is 8 bits, one can use the IRP bit in the status register for for 9 bit addressing. This gives an address space of 512. Access is accomplished thru the INDF register. Example: clear all registers from 0x20 to 0x7F

            movlw    0x20
            movwf    FSR
loop     clrf          INDF
            incf         FSR,F
            btfss        FSR,7
            goto        loop

3) Immediate Addressing
        Operand is in instruction. Typically , a 6 bit opcode and an 8 bit operand. Example:

                movlw    0xF3
                retlw        0xff

4) Absolute Address
         Will write directly to the PC. Examples include CALL and GOTO. 3 bit opcode and 11 bit absolute address

5) PC relative Address
        Add an 8 bit offset to the 13 bit PC. Ah....a problem. How to do this? First, write 5 bits to PCLATH (PC Latch High), next write 8 bits to PCL. The write to PCL will write PCLATH as well. Study example in video: 8 segment LED display.


A little bit of memory mapping:

Bank 0                           Bank 1        
0x00    indirect               0x80    indirect
0x01    TMR0                0x81    OPTION
0x02    PCL                   0x82    PCL
0x03    STATUS           0x83    STATUS
0x04    FSR                   0x84    FSR
0x05    PORTA             0x85    TRISA
0x06    PORTB             0x86    TRISB

Example code to set all of PORTB to output and turn on lights (note: you must have the proper include file for this to work)
	bsf 	STATUS,RP0		;select bank 1
   	movlw 	b'00000000'		;set PortB 8 outputs
   	movwf 	TRISA
	bcf	STATUS,RP0		;select bank 0
	movlw	0xff
	movwf	PORTB			; turn those lights on


According to the PIC data sheet, the chip we are using has

128 bytes of EEPROM Data Ram and 224 bytes of RAM. What's the difference ?