PIC2550 Clockcheck

Ein PIC 2550 soll mit 48MHz Taktfrequenz betrieben werden. Es wird ein 20MHz Quarzoszillator mit PLL verwendet. Als Programmierumgebung wird MPLAB X IDE mit XC8 C-Compiler eingesetzt.

Folgende Konfiguration wird verwendet:

<syntaxhighlight lang="MPASM" line start="100" enclose="div">

// PIC18F2550 Configuration Bit Settings

// 'C' source line config statements

1. include <xc.h>

// #pragma config statements should precede project file includes. // Use project enums instead of #define for ON and OFF.

// CONFIG1L

1. pragma config PLLDIV = 5 // PLL Prescaler Selection bits (Divide by 5 (20 MHz oscillator input))
2. pragma config CPUDIV = OSC1_PLL2// System Clock Postscaler Selection bits ([Primary Oscillator Src: /1][96 MHz PLL Src: /2])
3. pragma config USBDIV = 1 // USB Clock Selection bit (used in Full-Speed USB mode only; UCFG:FSEN = 1) (USB clock source comes directly from the primary oscillator block with no postscale)

// CONFIG1H

1. pragma config FOSC = HSPLL_HS // Oscillator Selection bits (HS oscillator, PLL enabled (HSPLL))
2. pragma config FCMEN = OFF // Fail-Safe Clock Monitor Enable bit (Fail-Safe Clock Monitor disabled)
3. pragma config IESO = OFF // Internal/External Oscillator Switchover bit (Oscillator Switchover mode disabled)

// CONFIG2L

1. pragma config PWRT = ON // Power-up Timer Enable bit (PWRT enabled)
2. pragma config BOR = OFF // Brown-out Reset Enable bits (Brown-out Reset disabled in hardware and software)
3. pragma config BORV = 3 // Brown-out Reset Voltage bits (Minimum setting)
4. pragma config VREGEN = OFF // USB Voltage Regulator Enable bit (USB voltage regulator disabled)

// CONFIG2H

1. pragma config WDT = OFF // Watchdog Timer Enable bit (WDT disabled (control is placed on the SWDTEN bit))
2. pragma config WDTPS = 32768 // Watchdog Timer Postscale Select bits (1:32768)

// CONFIG3H

1. pragma config CCP2MX = ON // CCP2 MUX bit (CCP2 input/output is multiplexed with RC1)
2. pragma config PBADEN = OFF // PORTB A/D Enable bit (PORTB<4:0> pins are configured as digital I/O on Reset)
3. pragma config LPT1OSC = OFF // Low-Power Timer 1 Oscillator Enable bit (Timer1 configured for higher power operation)
4. pragma config MCLRE = ON // MCLR Pin Enable bit (MCLR pin enabled; RE3 input pin disabled)

// CONFIG4L

1. pragma config STVREN = ON // Stack Full/Underflow Reset Enable bit (Stack full/underflow will cause Reset)
2. pragma config LVP = OFF // Single-Supply ICSP Enable bit (Single-Supply ICSP disabled)
3. pragma config XINST = OFF // Extended Instruction Set Enable bit (Instruction set extension and Indexed Addressing mode disabled (Legacy mode))

// CONFIG5L

1. pragma config CP0 = OFF // Code Protection bit (Block 0 (000800-001FFFh) is not code-protected)
2. pragma config CP1 = OFF // Code Protection bit (Block 1 (002000-003FFFh) is not code-protected)
3. pragma config CP2 = OFF // Code Protection bit (Block 2 (004000-005FFFh) is not code-protected)
4. pragma config CP3 = OFF // Code Protection bit (Block 3 (006000-007FFFh) is not code-protected)

// CONFIG5H

1. pragma config CPB = OFF // Boot Block Code Protection bit (Boot block (000000-0007FFh) is not code-protected)
2. pragma config CPD = OFF // Data EEPROM Code Protection bit (Data EEPROM is not code-protected)

// CONFIG6L

1. pragma config WRT0 = OFF // Write Protection bit (Block 0 (000800-001FFFh) is not write-protected)
2. pragma config WRT1 = OFF // Write Protection bit (Block 1 (002000-003FFFh) is not write-protected)
3. pragma config WRT2 = OFF // Write Protection bit (Block 2 (004000-005FFFh) is not write-protected)
4. pragma config WRT3 = OFF // Write Protection bit (Block 3 (006000-007FFFh) is not write-protected)

// CONFIG6H

1. pragma config WRTC = OFF // Configuration Register Write Protection bit (Configuration registers (300000-3000FFh) are not write-protected)
2. pragma config WRTB = OFF // Boot Block Write Protection bit (Boot block (000000-0007FFh) is not write-protected)
3. pragma config WRTD = OFF // Data EEPROM Write Protection bit (Data EEPROM is not write-protected)

// CONFIG7L

1. pragma config EBTR0 = OFF // Table Read Protection bit (Block 0 (000800-001FFFh) is not protected from table reads executed in other blocks)
2. pragma config EBTR1 = OFF // Table Read Protection bit (Block 1 (002000-003FFFh) is not protected from table reads executed in other blocks)
3. pragma config EBTR2 = OFF // Table Read Protection bit (Block 2 (004000-005FFFh) is not protected from table reads executed in other blocks)
4. pragma config EBTR3 = OFF // Table Read Protection bit (Block 3 (006000-007FFFh) is not protected from table reads executed in other blocks)

// CONFIG7H

1. pragma config EBTRB = OFF // Boot Block Table Read Protection bit (Boot block (000000-0007FFh) is not protected from table reads executed in other blocks)

</syntaxhighlight >

zum testen wird ein kurzes Testprogramm mit zyklischer Umschaltung PORTB Bit0 genutzt: <syntaxhighlight lang="C" line start="100" enclose="div"> void main(void) {

   ConfigureOscillator();
   portd_output;           // PortB pins Output

   while(1)
   {
    LATB = 0b00000000;     
    LATB = 0b00000001;    
   }

} </syntaxhighlight>

daraus entsteht folgender Assemblercode

<syntaxhighlight lang="MPASM" line start="100" enclose="div"> !void main(void) !{ ! ConfigureOscillator(); 0x7CEE: CALL 0x7CE2, 0 0x7CF0: NOP ! portd_output; // PortB pins Output 0x7CF2: MOVLW 0x0 0x7CF4: MOVWF TRISB, ACCESS ! while(1) ! { ! LATB = 0b00000000; 0x7CF6: MOVLW 0x0 // 1 Cy 0x7CF8: MOVWF LATB, ACCESS // 1 Cy ! LATB = 0b00000001; 0x7CFA: MOVLW 0x1 // 1 Cy 0x7CFC: MOVWF LATB, ACCESS // 1 Cy 0x7CFE: BRA 0x7CF6 // 2 CY </syntaxhighlight>

ein am entsprechenden Pin angeschlossenes MSO zeigt nun folgendes:

Fazit

Was ist daraus zu erkennen?!

48 MHz Taktfrequenz bedeutet eine Zyklusfrequenz Takt /4 ==> 12 MHz, die Programmschleife besteht aus 6 Zyklen was 24 Taktzyklen entspricht.
12 MHz Zyklusfrequenz ergibt eine Zykluszeit von 1/12MHZ = 83,33 ns (Ausführung eines 1 Zyklusbefehls dauert 83,3ns).
Die Low-Zeit ergibt sich aus 2 Cy die High-Zeit sind die restlichen 4 Cy. Die Periodendauer ist somit 6*83,3ns = 499,999ns was wiederum einer Frequenz von 2MHz entspricht.
Da diese Werte messtechnisch nachgewiesen wurden scheint die Konfiguration erfolgreich durchgeführt worden zu sein.