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UNIT 8

UNIT 8. Keypad Interface Contact Closure Counter Exceptions (Interrupts and Reset). Keypad Figure 5.5. Use as Outputs. Use as Inputs. Interface Keypad to Port H. Port H[3:0] – Output Full Drive Port H[7:4] – Input Pulls are Optional since Pull-ups on Keypad Disable Interrupts.

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UNIT 8

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  1. UNIT 8 Keypad Interface Contact Closure Counter Exceptions (Interrupts and Reset)

  2. Keypad Figure 5.5 Use as Outputs Use as Inputs

  3. Interface Keypad to Port H • Port H[3:0] – Output • Full Drive • Port H[7:4] – Input • Pulls are Optional since Pull-ups on Keypad • Disable Interrupts

  4. Port H Definitions /* Port H Port Definitions */ #define PTH _P(0x260) #define PTIH _P(0x261) #define DDRH _P(0x262 #define RDRH _P( 0x263 #define PERH _P( 0x264 #define PPSH _P( 0x265 #define PIEH _P( 0x266 #define PIFH _P( 0x267

  5. Port H Initialization /* Direction */ DDRH = 0x0F; /* Full Drive */ RDRH = 0x00; /* Pulls Not Required since pull resistors on keypad */ PERH = 0x00; /* Disable Interrupts */ PIEH = 0x00; /* Unselect Rows in Keypad */ PTH = 0x0F;

  6. Procedure to Read Keypad /* if key is pressed, return ‘0’, ‘1’, …, ‘F’ else ‘z’ */ unsigned char keypress(void) {unsigned char mask[4]=0x0E,0x0D,0x0B,0x07; //1110, 1101, 1011, 0111 int i,j; char key; for(key=‘z’,i=0;i<4;i++) {PTH = mask[i]; // assert a 0 on a row in keypad for(j=0;j<4;j++) {if((PTH>>4) == mask[j]) //check each column for a returned 0 {if (((i<<2)+j)<10) key = ‘0’+(i<<2)+j; //value = 4*row# + column# else key=‘A’+(i<<2)+j -10;} //different offset for values>9 } } PTH = 0x0F; // unselect row return (key); }

  7. Algorithm Implemented in C i=0 PTH[3:0] = 0xE // Row 0 j=0 PTH[7:4] = 0xE key=‘0’ j=1 PTH[7:4] = 0xD key=‘1’ j=2 PTH[7:4] = 0xB key=‘2’ j=3 PTH[7:4] = 0x7 key=‘3’ i=1 PK[3:0] = 0xD // Row 1 j=0 PTH[7:4] = 0xE key=‘4’ j=1 PTH[7:4] = 0xD key=‘5’ j=2 PTH[7:4] = 0xB key=‘6’ j=3 PTH[7:4] = 0x7 key=‘7’ i=2 PTH[3:0] = 0xB // Row 2 j=0 PTH[7:4] = 0xE key=‘8’ j=1 PTH[7:4] = 0xD key=‘9’ j=2 PTH[7:4] = 0xB key=‘A’ j=3 PTH[7:4] = 0x7 key=‘B’ i=3 PTH[3:0] = 0x7 //Row 3 j=0 PTH[7:4] = 0xE key=‘C’ j=1 PTH[7:4] = 0xD key=‘D’ j=2 PTH[7:4] = 0xB key=‘E’ j=3 PTH[7:4] = 0x7 key=‘F’ keycode = 4*i+j; If(keycode<10) key = ‘0’ + keycode; else key = ‘A’ + keycode – 10;

  8. Another ProcedureRead Keypad /* if key is pressed, return ‘0’, ‘1’, …, ‘F’ else ‘z’ */ unsigned char keypress(void) {unsigned char mask[16]=0xEE,0xDE,0xBE,0x7E, 0XED,0xDD,0xBD,0x7D, 0xEB,0xDB,0xBB,0x7B, 0xE7,0xD7,0xB7,0x77; int i; char key; for(key=‘z’,i=0;i<16;i++) {PTH = mask[i]; // assert row in keypad if(PTH == mask[i]) {if (i<10) key = ‘0’+i; else key=‘A’+i-10;} } PTH = 0x0F; // unselect row return (key); } //SEE NEXT PAGE

  9. sent out Row 0 EE 1110 1110-------key = 0 DE 1101 1110-------key = 1 BE 1011 1110-------key = 2 7E 0111 1110-------key = 3 Row 1 ED 1110 1101-------key = 4 DD 1101 1101-------key = 5 BD 1011 1101-------key = 6 7D 0111 1101-------key = 7 Row 2 EB 1110 1011-------key = 8 DB 1101 1011-------key = 9 BB 1011 1011-------key = A 7B 0111 1011-------key = B Row 3 E7 1110 0111-------key = C D7 1101 0111-------key = D B7 1011 0111-------key = E 77 0111 0111-------key = F

  10. Keypads - Section 5.2.3 • Example Uses Port B and not Port H • Uses switch() statements rather than for loops.

  11. Another I/O Example: Contact Closure Counter Connect 8 Push-Button to the Pins of Port H Push-Button is normally open => “1” is input Press Momentary Push-Button => “0” is input Count Number of Times a Push-Button is Pressed Count “1” to “0” Transitions on each Pin

  12. /* port definitions */ int main() {int count[8] = 0,0,0,0,0,0,0,0; int sum=0, int i; unsigned char oldsw, newsw, change; /* init port h */ oldsw = PTH; newsw = PTH; while(sum<1000) { while(oldsw == newsw) newsw = PTH; change = (oldsw^newsw) & oldsw; oldsw = newsw; for(i=0, i<8, i++) if(((change>>i)&0x01)!=0) {count[i]++; sum++} } return(0); } //count only 1->0 transitions

  13. Details • Old = 0x23 = 0b00100011 • New = 0x32 = 0b00110010 • Old ^ New = 0b00010001 • (Old ^ New) & Old = 0b00000001 • Increment count[0]

  14. Exceptions, Interrupts, and Resets

  15. “Exception”: A break in normal program flow, normally initiated by some hardware condition.

  16. Two types of exceptions: • Resets • Interrupts

  17. Exceptions can be initiated by either Internal or external events • When exceptions occurs, the microcontroller: • - Uses an address from a Vector Table to get to a Service Routine. • - Saves the processor state on the stack (for Interrupts)

  18. Events that Can Cause a Reset: • External Reset (via active low reset pin) • Power-on Reset • Computer Operating Properly (COP) Reset • Clock Monitor Reset • Disabled by Debugger in Lab

  19. Computer Operating Properly(COP Reset) • Also called Watch Dog Timer • User-configurable countdown timer • If timer reaches 0, a system reset is triggered • User must repeatedly reset using a reset timer sequence • Disabled in Lab

  20. Clock Monitor Reset • Clock Monitor Reset when system clock frequency drops below a prescribed value or stops

  21. Two Classes of Interrupts: • Nonmaskable Interrupts (cannot be disabled by software) • Maskable Interrupts • Masked – Disabled (By software) • Unmasked – Enabled (By software)

  22. Nonmaskable Interrupts • X bit – Condition Code Register (CCR) • X=“1” – Disables all Interrupts • X=“0” – Enable all Interrupts • During Normal System Reset, X=“1” • After System Initialization, X=“0” by software • X can not be set to “1” by software

  23. Types of Nonmaskable Interrupts • Nonmaskable Interrupt Request (/XIRQ) • /XIRQ is External Pin • /XIRQ Active Low Generates Request • Unimplemented Instruction Trap • Software Interrupt Instruction (SWI)

  24. Enabling of Maskable Interrupts • I bit in Condition Code Register (CCR) • I = “0” Enable Maskable Interrupts • I = “1” Disable Maskable Interrupts • C Program Instructions • ENABLE(); – Sets I=“0” • DISABLE(); – Sets I=“1”

  25. Maskable Interrupt Sources • IRQ Pin • Real Time Clock Interrupt • Enhanced Capture Timer Channels • Pulse Accumulators • SPI and SCI Serial I/O • A/D System • Port J, H, and P Pins • Network Serial I/O • Other

  26. Interrupt Initialization (Fig. 4.21)

  27. Interrupt Vector Table • 2 Bytes Allocated for each Interrupt Source • Contents of Vector • Starting Address of Interrupt Service Routine • Vectored System • Each Source has a dedicated Vector • “Boost” priority of one maskable interrupt to highest priority (HPRIO Register)

  28. Interrupt Handling – Fig. 4.21

  29. Interrupt Handling • Save State of CPU on Stack by Hardware • Disable Masked Interrupt System • Determine Interrupt Source by Hardware • Transfer Control to Interrupt Handling Software by Hardware • Service Interrupt Request by Interrupt Service Routine (ISR) – User written • Restore State of CPU from Stack using RTI instruction

  30. Why Interrupts? • Most operating systems support multiprocessing – Several application running at same time. • Desirable to eliminate “busy waits” in all software since busy waits can monopolize CPU resources.

  31. Recall: Parallel Printer Interface(Not using interrupts) Busy Printer /Strobe Busy Data[7:0] Yes No Write Data Strobe

  32. Print a Character on Printer void printc(unsigned char c) { /* Wait while Printer is Busy */ while((PTJ&0x02)!=0); // BUSY WAIT /* Printer not Busy, Output Data */ PTH = c; /* Generate Strobe * PTJ = PTJ & 0xFE; // Bit 0 Low PTJ = PTJ | 0x01; // Bit 0 High }

  33. Printer Waveforms Busy Data /Strobe ACK PC Parallel Printer Port Definition www.fapo.com/ieee1284.htm

  34. Parallel Printer Interface Busy Printer /Strobe Busy Data[7:0] ACK Yes No Write Data Strobe

  35. Interfacing a Parallel Printer Using Interrupts • Present data to printer • Send Strobe signal to printer • When printer is no long busy, it sends out an ACK pulse to request an interrupt • (The interrupt indicates the printer is ready to receive a new character to print.)

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