Analog Devices AN765 Application Notes

AN-765

ELMT

PLA

INPUT

PLAIRQ0

PLAIRQ1

START

PLAIRQ0?

PLAIRQ1?

PLAIRQ0 ISR

STARTTIMER1

DISABLE PLAIRQ0

ENABLE PLAIRQ1

PLAIRQ1 ISR READ T1CAP

DISABLE PLAIRQ1

STOP TIMER1

ENABLE PLAIRQ0

YES

APPLICATION NOTE

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Measuring Duration of a Short Pulse on the ADuC702x Family

by Aude Richard

INTRODUCTION

The ADuC702x has four external interrupts which are congurable only as level triggered and active low. Therefore measuring a short pulse using the external interrupts would require some combination of external glue logic.

This application note describes a method to allow the measurement of a short pulse duration (a few milliseconds) using timer1 and the PLA. This technique does not require any external digital logic.

CONCEPT

The PLA (programmable logic array) can be described as glue logic; its function is to remove the requirement for simple external logic. It consists of 16 elements, each element containing a two-input lockup table that can be congured to generate any logic function based on one or two inputs.

The PLA can be routed to the internal interrupt system and has two dedicated interrupt bits in the interrupt controller.

Timer1 is a general-purpose timer with extra features, such as a capture event mode. Timer1 capture register (T1CAP) can be triggered by a selected interrupt source. This feature can be used to determine the start of an event with more accuracy than if a timer was triggered upon entry to the ISR. The capture event feature is used in this application note.

HARDWARE CONSIDERATION

Any of the GPIO available as PLA input can be used to measure the pulse. This method uses two elements to utilize two interrupt sources, as shown in Figure 1.

Output of element x is fed back to element y, congured as a “not” gate. The output of element y is congured to trigger PLAIRQ1 when the input signal goes back low.

SOFTWARE

Everything is done in the interrupt service routine, as shown in Figure 2.

Figure 2. Flowchart

Timer1 is started in PLAIRQ0 ISR. PLAIRQ0 is disabled to ensure the ISR cannot be re-entered and PLAIRQ1 is enabled.

In the PLAIRQ1 ISR, the value captured automatically by timer1 is read in T1CAP. PLAIRQ1 is disabled to ensure the ISR cannot be re-entered and the PLAIRQ0 is re-enabled to allow a new measurement. Timer1 also needs to be stopped and reset.

See the source code provided.

Figure 1. PLA Conguration

The pulse is passed through element x. The output of element x is congured to trigger PLAIRQ0 when its input goes high.

REV. 0

LIMITATION AND PRECISION

PLAIRQ0 interrupt introduces a delay to start timer1. Interrupt latency on the ADuC702x is between 5 and 50 processor cycles, which corresponds to just over 1.1 s in the example (using a continuous 45 MHz processor clock). Therefore, this method should only be used for pulses in the millisecond range and greater.

AN-765

#include<aduc7020.h>

long pulse;

void My_IRQ_Function(void); // IRQ Function Prototype

int main (void) {

IRQ = My_IRQ_Function; // Specify Interrupt Service Routine

PLAELM0 = 0x0035; // pass PLAELM1 = 0x0047; // not PLAIRQ = 0x1110; //

IRQEN = 0x080000; // enable PLA IRQ0

while (1){ } }

/********************************************************************/ /* */ /* Interrupt Service Routine */ /* */ /********************************************************************/

AN05319–0–2/05(0)

void My_IRQ_Function() { if ((IRQSTA & PLA_IRQ0_BIT) == 0x00080000) // PLAIRQ0 { T1CON = 0x32180; // start Timer1. capture PLAIRQ1 IRQCLR = 0x80000; // disable PLA IRQ0 IRQEN = 0x00100000; // enable PLA IRQ1 } if ((IRQSTA & PLA_IRQ1_BIT) == 0x00100000) // PLAIRQ1 { pulse = T1CAP; // read the capture event IRQCLR = 0x00100000; // disable PLA IRQ1 IRQEN = 0x80000; // enable PLA IRQ0 T1LD = 0x00; // to reset timer1 T1CON = 0xC0; // reset timer1 T1CON = 0; // stop timer1 } return ; }

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