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MicroChip 3

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  1. MicroChip 3 UNCA ECE 406 April 18, 2007

  2. C variable view int state ; int nxtState ; int oldRST ; int oldDIN ; int oldCLK ; volatile int RST ; volatile int DIN ; volatile int CLK ; int DOUT ;

  3. C initialization /* many of these choices are arbitrary */ nxtState = state = 0 ; oldRST = 0 ; oldDIN = 0 ; oldCLK = 0 ;

  4. C Loop while (1) { if (CLK != oldCLK) processClockTrans() ; if (RST != oldRST || DIN != oldDIN) processDataTrans() ; }

  5. Process Clock Transaction processClockTrans() { oldCLK = CLK ; if (CLK) { state = nxtState ; processDataTrans() ; } }

  6. Process Data Transaction processDataTrans() { oldRST = RST ; oldDIN = DIN ; if (RST) nxtState = 0 ; else { nxtState = (2*state+DIN)%3 ; DOUT = (state==0 && DIN==0) || (state==2 && DIN==1) ; }

  7. Action at reset • Program starts at “reset vector” • goto FSM • Loading the OSCAL [p 21] • “pre-programmed internal callibration value” • Location FFh contains callibration value • Loaded into W before reset • Suggest loading at first instruction • FSM movwf OSCAL

  8. Things that might be done • Examine STATUS to determine how reset • Could awaken on “sleep on pin change” • Register 03h • Set OPTION to allow wakeup • On pin change or reset • But how to avoid resetting the registers • Requires use of OPTION instruction

  9. Setting I/O direction • TRIS register • Input (high-impedance) – 0 • Output – 1 • We need xxxx1011 (0Ah) • Requires TRIS instruction • movlw 0Ah • tris GPIO • The GPIO does seem odd

  10. Setting the initial output • Not really specified in the problem • GPIO register • Register 06h • Buffer for I/O • Can read an output pin • bcf GPIO,BOUT

  11. Initialize registers clrf STATE clrf NXTST clrf OLDRST clrf OLDDIN clrf OLDCLK

  12. Start the loop • Check for a CLK transition FSMLP movf GPI0,W andlw fdh (~BCLK) xorf OLDCLK,W btfsc STATUS,Z goto CLKTRANS

  13. Check for RST transition movf GPI0,W andlw f7h (~BRST) xorf OLDRST,W btfsc STATUS,Z goto DATTRANS goto FSMLP

  14. Check for DAT transition movf GPI0,W andlw feh (~BDAT) xorf OLDRST,W btfsc STATUS,Z goto DATTRANS

  15. Start the loop • Check for a CLK transition FSMLP movf GPI0,W andlw fdh (~BCLK) xorf OLDCLK,W btfsc STATUS,Z goto CLKTRANS

  16. Clock transition CLKTRANS movf GPIO,W andlw BCLK movwf OLDCLK btfsc STATUS,Z goto FSMLP movf NXTST,W movwf STATE goto DATTRANS • Only update STATE here!

  17. Data transition -- save old DATTRANS movf GPIO,W andlw BDAT movwf OLDDAT movf GPIO,W andlw BRST movwf OLDRST

  18. Reset check btfsc STATUS,Z goto NOTRST clrf NXTST goto FSMLP

  19. Next state and output decf STATE,W btfsc STATUS,C goto ST0TRANS btfsc STATUS,Z goto ST1TRANS goto ST2TRANS

  20. State 0 transition ST1TRANS clrf NXTST btfsc GPIO,BDIN goto ST1D1TRANS bsf GPIO,BDOUT goto FSMLP ST1D1TRANS incf NXTST,F bsc GPIO,BDOUT goto FSMLP

  21. Arithmetic approach DATTRANS movf STATE,W addf STATE,W btfsc GPIO,BDAT addlw 1 movwf NXTST sublw 3 btfsc STATUS,C movwf NXTST

  22. Table approach – part 1 movf STATE,W addf STATE,W btfsc GPIO,BDAT addlw 1 call TBLLKUP

  23. Table lookup TBLLKUP addlw NXTSTTAB movf PCL NXTSTTAB retlw 10h retlw 01h retlw 02h retlw 00h retlw 01h retlw 12h

  24. Table approach – part 2 call TBLLKUP movwf NXTST btfsc NXTST,4 goto SETOUT1 bcf GPIO,BDOUT goto FSMLP SETOUT1 bcf NXTST,4 bsf GPIO,BDOUT goto FSMLP

  25. Perils of PIC Assembly • Quirky instruction set • Quirky special registers • Quirky addressing • Quirky initialization • Variations with processors

  26. Data memory – PIC18 • Data memory addressed by 12 bits • Most instructions can address 8 bits • BSR, Bank Select Register • Provides access to addition sets • But must be set before access • Processors vary in number of banks • Similar to segments in x8086 • Leads to “far” and “near” pointers

  27. Program memory – PIC 18 • Program memory address by 22 bits • PC is 21 bits • Last bit is always 0 • Held in three registers • Only PCL (low 8 bits) are directly accessable • Use PCLATH to write other two • Program memory can be read/written • While running

  28. Program branching • GOTO and CALL instructions • Use two “words” of program memory • Holds 20 bits of PC • Entire 2-Mbyte memory range • BRA (unconditional) instruction • 11 bit offset (actually 12) of PC • BC (conditional) instruction • 8 bit offset (actually 9) from PC

  29. C compilation • Huge criticism of PIC • Too many variations • Unusual memory struction • No good stack support • Addressing relative to stack pointer • But the x8086 wasn’t that go either

  30. C compilation • Not bad with the right chip • Pin I/O • With TRISx and PORTx variables • Some printf • Through UARTS • Interrupt handlings • Hard working optimization

  31. PIC examples • MadLab • PIC Source • MicroChip Application Notes • John Hopkins ECE448