AT89C External timer inputs do not work as regular inputs

[Joris Dallaire]

Hello,

 

First post here, from a newbie hobbyist.

 

I have an atmel AT89C2051 and i can’t get the external timer inputs (T0 and T1) to work as regular inputs. I write 1s to the whole P3 port and all available pins work as expected except those two.

 

I do use the timers, and they are set in TMOD as timer mode (not counter), so the pins should be available as regular input pins, no?

 

Any help appreciated.

Joris

[SHAH DISHANT H.]

Hi,

 

Ya what you said is perfectly fine and you must get those pins as GPIO.

 

Can you post ur code so that we can verify ?

[Joris Dallaire]

Hello,

 

Thank you for the helping hand! Here’s the code. There is a debounce routine in there, but basically when any button connected to P3 is pushed the “State” variable becomes greater than 1. Here the PB_UP and PB_DOWN buttons are set to pins 1 and 0 and it works, but it doesn’t when set to T0 and T1.

 

Thanks you in advance for your advice.

 

__sfr __at (0x90)   P1;

__sfr __at (0xB0)   P3;

__sfr __at (0x89)   TMOD;

__sfr __at (0x8A)   TL0;

__sfr __at (0x8C)   TH0;

__sbit __at (0x8C)  TR0;

__sfr __at (0x8B)   TL1;

__sfr __at (0x8D)   TH1;

__sbit __at (0x8E)  TR1;

__sbit __at (0xAF)  EA;     /* 0=Disable all interrupts */

__sbit __at (0xA9)  ET0;        /* 1=Enable Timer 0 interrupt */

__sbit __at (0xAB)  ET1;        /* 1=Enable Timer 0 interrupt */

__sbit __at (0x8F)  TF1;

 

#define PB_DOWN 0 // Bit position of switch in input port

#define PB_UP 1 // Bit position of switch in input port

#define MAX_CHECKS 10 // # checks before a switch is debounced

volatile unsigned char  Debounced_State = 0; // Debounced state of the switches

volatile unsigned char  State[MAX_CHECKS]; // Array that maintains bounce status

volatile unsigned int   Index; // Pointer into Switch state array

volatile unsigned char  selectedPosition = 0;

static unsigned char    maxPosition = 7;

volatile unsigned char  freezeLoops = MAX_CHECKS*2; // Number of loops that the switches are frozen after a press is detected

volatile unsigned char  currentFreezeLoop; // Holds the countdown of loops during the frozen state

 

unsigned char getBtnStatus(unsigned char bitPosition) {

    // Bit extraction: (x >> lsb) & ~(~0 << (msb-lsb+1))

    return (Debounced_State >> bitPosition) & ~(~0 << 1);

}

 

 

void updateIndex() {

    if (getBtnStatus(PB_DOWN) && selectedPosition > 0) selectedPosition--;

    if (getBtnStatus(PB_UP) && selectedPosition < maxPosition) selectedPosition++;

    TF1 = 1;  /* Force Timer 1 overflow to provoque ISR execution */

}

 

// Service routine called by a timer interrupt

void debounceSwitches() {

    unsigned int i=MAX_CHECKS, j=0xFF;

    State[Index]=~P3; //Load status of all switches connected to P3 in a state byte. Inverted switch logic so that a swtch press (0 volts) equals a logic 1

    ++Index;

    do { j = j & State; } while (--i);

    if (Debounced_State != j)  { /* Change of switch state */

        Debounced_State = j;

    }

 

    if (Debounced_State > 0 && !currentFreezeLoop ) { /* One of P3 pins has changed and the port is "unfrozen" */

        currentFreezeLoop = freezeLoops; /* Reset frozen state counter */

        updateIndex();

    }

    if (currentFreezeLoop) currentFreezeLoop--; { /* frozen state, decrement frozen state counter */

    }

 

    if ( Index >= MAX_CHECKS ) Index=0;

}

 

void ext0(void) __interrupt 0 __using 0 { // External Interrupt 0: Highest priority ISR

}

 

void intT0(void) __interrupt 1 __using 1 { // Timer 0 Over?ow: Second highest priority ISR, scans input buttons

    debounceSwitches();

 

    TL0=0xDF; //Load value of 45535 into timer, leaving 20000 ticks ( approx. 10 ms) before next overflow

    TH0=0xB1;

}

 

void ext1(void) __interrupt 2 __using 2 { // External Interrupt 1: Third highest priority ISR

}

 

void intT1(void) __interrupt 3 __using 3 { // Timer 1 Over?ow: Second highest priority ISR

    P1=~(1 << selectedPosition);

}

 

main() {

    P3=0xFF;        /* Intitalize all pins of P3 as inputs */

 

    TMOD|=0x1;      /* Timer 0 Mode 1 */

//  TMOD|=0x11;     /* Both timers in Mode 1 */

    TH0=0;

    TL0=0;

    TL0=0xDF; //Load value of 45535 into timer, leaving 20000 ticks ( approx. 10 ms) before next overflow

    TH0=0xB1;

    TR0=1; /* Start Timer 0 */

    ET0=1; /* Enable Timer 0 */

 

    TL1=0;

    TH1=0;

//      TR1=1; /* Start Timer 0 */

    ET1=1; /* Enable Timer 0 */

    /* Bit or byte addressable. If EA is 0, all interrupts are disabled.

                If EA is 1, any interrupt is enabled by setting its bit to 1:

                – EA, IE.7: if EA=0, neither interrupt will be acknowledged. If

                EA=1, each interrupt source may be individually enabled or disabled.

                – ET2, IE.5: timer 2 overflow

                – ES, IE.4: interrupt serial port

                – ET1, IE.3: Timer 1 overflow

                – EX1, IE.2: external interrupt 1

                – ET0, IE.1: Timer 0 overflow

                – EX0, IE.0: external interrupt 0

                */

    EA=1;           /* global interrupt enable */

    

    /* TCON register

    • Bit or byte addressable. Bit functions:

    – TF1, TCON.7: Flag Timer 1 overflow. Hardware erased when interrupt routine is running

    – TR1, TCON.6: Timer 1 control bit. if TR1=1, Timer 1 ON. if TR1=0, Timer1 OFF.

    – TF0, TCON.5: Flag Timer 0 overflow. Same as Flag Timer 1 overflow.

    – TR0, TCON.4: Timer 0 control bit.Same as for Timer1.

    – IE1, TCON.3: Flag for external interrupt 1. Set hardware when external interrupt 1 edge detected; hardware reset when processing interrupt.

    – IT1, TCON.2: Interrupt 1 type control bit. If IT1=1, interrupt 1 is triggered

    by a falling down edge. If IT1=0, interrupt 1 is triggered by level 0.

    – IE0, TCON.1: Flag for external interrupt 0. Set hardware when interrupt edge detected, hardware erased when interrupt processing. */

    updateIndex(); // Load initial relay conditions (selected position 0, mute )

    while (1);

}

[Ashutosh Bhatt]

there are two timer/counter in 8051

at a time it can work as timer or counter

if T0 you are configuring as timer. it will count internal pulses at 1 MHz

else if you are configurating T0 as counter it will count external pulses (max at 1 MHz)

 

similar for T1. no need to configure anything else. 

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