Introduction

In an effort to increase the number of times per second the loop function is called when using analogRead functions non-blocking equivalent functions have been implemented into the chipKIT-core.

The existing analogRead function does three things: <ol> <li>Select ADC channel and start conversion.</li> <li>Do nothing while waiting for conversion to complete.</li> <li>Read converted value and return it to user.</li> </ol>

The “do nothing” part is what we aim to eliminate this non-blocking implementation and is indeed what we have done. Three newly exposed functions [analogReadConversionStart(pin), analogReadConversionComplete(), analogReadConversion()] implement and the analogRead() in wiring_analog.c simply becomes this:

int analogRead(uint8_t pin)
{
    analogReadConversionStart(pin);
    while( ! analogReadConversionComplete() );
    return analogReadConversion();
}

Results

The result of using the non-blocking analogRead functions result in a simple loop speed up of nearly 2x. On a MZ EFG chip this speed up is about 4.9x as fast as the blocking functions and allows for a loop frequency greater than 1MHz!

chipKIT Board (Rev)	PIC32	Clock Freq	loop() Frequency Blocking Analog Read	loop() Frequency Non-Blocking Analog Read	Improvement	Notes
Lenny	MX	40 Mhz	49.14 kHz (20.35uS)	127.39 kHz (7.84uS)	2.59
Fubarino SD (1.5)	MX	80 Mhz	90.91 kHz (10.99uS)	217.39 kHz (4.60uS)	2.39
Wi-Fire (C)	MZEFG	200 Mhz	233.10 kHz (4.29uS)	1162.79 kHz (0.86us)	4.98
Wi-Fire (B)	MZECG	200 Mhz	405.63 Hz (2465.3uS)	N/A	N/A	Non-Blocking Not Supported

Example

The following code example shows how to use the non-blocking analogRead functions. In the current implementation it is important that you do not mix blocking and non-blocking analogRead functions due to an internal state machine that keeps track of which ADC channel is currently selected.

This example is used to benchmark the performance of the non-blocking and blocking analog read functions and will print the results to the serial monitor. The default mode is Non-Blocking. To switch to Blocking ADC for comparison simply comment out #define FAST_ADC.

// This is in wiring.c, we shouldn't have to redefine it here (issue #288)
#define CORETIMER_TICKS_PER_MICROSECOND    (F_CPU / 2 / 1000000UL)

//#define ADC_TEST_PIN A12
#define ADC_TEST_PIN 16

void setup() {
    // put your setup code here, to run once:
    Serial.begin(115200);           // start serial for output
    delay(3000);
    Serial.println("chipKIT-core ADC speed test");
#define FAST_ADC
#ifdef FAST_ADC
    analogReadConversionStart(ADC_TEST_PIN);
#endif
}

void loop() {
    // put your main code here, to run repeatedly:
    static uint32_t start_us = readCoreTimer();
    static uint32_t stop_us = readCoreTimer();

    uint32_t value;

	#ifdef FAST_ADC
		if ( analogReadConversionComplete() ) {
		value = analogReadConversion();
			analogReadConversionStart(ADC_TEST_PIN);
		}
	#else
		value = analogRead(ADC_TEST_PIN);
	#endif

    stop_us = readCoreTimer(); //micros();

    // Display Loop Time
    Serial.print("ADC_TEST_PIN: ");
    Serial.print(value, DEC);
    Serial.println("");
    Serial.print("F_CPU: ");
    Serial.print(F_CPU, DEC);
    Serial.print(" ");
    float us = (float)(stop_us - start_us) / (float)CORETIMER_TICKS_PER_MICROSECOND;
    Serial.print(us, 3);
    Serial.print(" us, ");
    Serial.print(1.0 / us * 1000000.0, 3);
    Serial.print(" Hz");
    Serial.println("");
    delay(1000);

    start_us = readCoreTimer(); //micros();

}

Non-Blocking Analog Read Functions

Introduction

Results

Example