Erik Olieman / FastAnalogIn

Class similar to AnalogIn that uses burst mode to run continious background conversions so when the input is read, the last value can immediatly be returned.

Dependents:   KL25Z_FFT_Demo test_armmath KL25Z_FFT_Demo_tony KL25Z_FFT_Demo_tony ... more

Supported devices

  • LPC1768
  • LPC4088
  • LPC11u24
  • KLxx
  • K20D50M

Introduction

When you read an AnalogIn object it will enable the corresponding ADC channel, depending on the implementation do either one or multiple measurements for more accuracy, and return that value to your program. This way the ADC is only active when it is required, and it is fairly straightforward. However the downside is, is that an ADC is relatively slow. On the LPC1768 it runs at 200kHz -> in that time it could also have done 500 instructions.

FastAnalogIn

This library uses the 'burst' feature of the microcontroller. This allows the ADC on the background to perform the AD conversions without requiring intervention from the microcontroller's core. Also there are no interrupts used, so also your time-sensitive code is not affected.

What the burst feature does is check which AD-channels are enabled, and he converts the enabled AD-channels one at a time. The result he stores in a register, where each channel has its own register. So this library checks which pins are used (you may make several FastAnalogIn objects, both for different pins and for the same pin, generally not extremely useful, but it is supported), and enables the relevant channels.

Reading a pin is done exactly the same for the user as AnalogIn, the read and read_us functions both work the same, and also the float operator is supported. However now it doesn't have to start a new conversion, so minus some overhead it can almost directly return the last measured value, no need to wait on the ADC!

Enable/Disable

FastAnalogIn has a few extra options that normal AnalogIn does not have: specifically you can either choose to have a FastAnalogIn object enabled or disabled. This is done with either the enable(bool enabled) and disable() functions, where enable(false) is equal to disable(), or by adding a second true/false argument to the constructor to either have it enabled at the beginning or disabled. By default it will be enabled.

LPC1768 & LPC4088
When a FastAnalogIn object is enabled, its corresponding ADC channel is also being scanned by the ADC and so it works as described above. When it is disabled you can still use the read functions, only now it will only enable the ADC channel for one conversion (actually two since for some reason the first conversion seems a bit weird), and when that conversion is done it will disable it again.

Since the ADC has to do the conversions one channel at a time, it becomes slower per channel if you enable many channels. For example, if you want to sample a sensor at a very high rate, and you also want to monitor your battery voltage. Then there is no reason to run an AD conversion on your battery continiously, so you can disable that channel and only run it once in a while.

KLxx
Multiple Fast instances can be declared of which only ONE can be continuous (all others must be non-continuous).
Example:

FastAnalogIn   speed(PTC2);           // Fast continuous
FastAnalogIn   temp1(PTC2, 0);        // Fast non-continuous.
FastAnalogIn   temp2(PTB3, 0);        // Fast non-continuous.

Downsides

Of course there are always downsides present. The extra power consumption probably won't be relevant for most, but still it is there. Aditionally there is no median filter like the normal AnalogIn has. Finally if you use AnalogIn you know exactly when the conversion happened, with FastAnalogIn you only know it was recently done but not exactly when.

AnalogIn + FastAnalogIn

Don't run both AnalogIn and FastAnalogIn objects in parallel as the results are unpredictable.
Both objects modify microcontroller registers, and neither of them bothers to inform the other one.
That's also the reason the disable() function was added.

FastAnalogIn.h

Committer:
Sissors
Date:
16 months ago
Revision:
12:46fbc645de4d
Parent:
9:31184aa1449c

File content as of revision 12:46fbc645de4d:

#ifndef FASTANALOGIN_H
#define FASTANALOGIN_H

/*
 * Includes
 */
#include "mbed.h"
#include "pinmap.h"

#if !defined TARGET_LPC1768 && !defined TARGET_KLXX && !defined TARGET_LPC408X && !defined TARGET_LPC11UXX && !defined TARGET_K20D5M && !defined TARGET_LPC11XX
    #error "Target not supported"
#endif

 /** A class similar to AnalogIn, only faster, for LPC1768, LPC408X and KLxx
 *
 * AnalogIn does a single conversion when you read a value (actually several conversions and it takes the median of that).
 * This library runns the ADC conversion automatically in the background.
 * When read is called, it immediatly returns the last sampled value.
 *
 * LPC1768 / LPC4088
 * Using more ADC pins in continuous mode will decrease the conversion rate (LPC1768:200kHz/LPC4088:400kHz).
 * If you need to sample one pin very fast and sometimes also need to do AD conversions on another pin,
 * you can disable the continuous conversion on that ADC channel and still read its value.
 *
 * KLXX
 * Multiple Fast instances can be declared of which only ONE can be continuous (all others must be non-continuous).
 *
 * When continuous conversion is disabled, a read will block until the conversion is complete
 * (much like the regular AnalogIn library does).
 * Each ADC channel can be enabled/disabled separately.
 *
 * IMPORTANT : It does not play nicely with regular AnalogIn objects, so either use this library or AnalogIn, not both at the same time!!
 *
 * Example for the KLxx processors:
 * @code
 * // Print messages when the AnalogIn is greater than 50%
 *
 * #include "mbed.h"
 *
 * FastAnalogIn temperature(PTC2); //Fast continuous sampling on PTC2
 * FastAnalogIn speed(PTB3, 0);    //Fast non-continuous sampling on PTB3
 *
 * int main() {
 *     while(1) {
 *         if(temperature > 0.5) {
 *             printf("Too hot! (%f) at speed %f", temperature.read(), speed.read());
 *         }
 *     }
 * }
 * @endcode
 * Example for the LPC1768 processor:
 * @code
 * // Print messages when the AnalogIn is greater than 50%
 *
 * #include "mbed.h"
 *
 * FastAnalogIn temperature(p20);
 *
 * int main() {
 *     while(1) {
 *         if(temperature > 0.5) {
 *             printf("Too hot! (%f)", temperature.read());
 *         }
 *     }
 * }
 * @endcode
*/
class FastAnalogIn {

public:
     /** Create a FastAnalogIn, connected to the specified pin
     *
     * @param pin AnalogIn pin to connect to
     * @param enabled Enable the ADC channel (default = true)
     */
    FastAnalogIn( PinName pin, bool enabled = true );
    
    ~FastAnalogIn( void )
    {
        disable();
    }
    
    /** Enable the ADC channel
    *
    * @param enabled Bool that is true for enable, false is equivalent to calling disable
    */
    void enable(bool enabled = true);
    
    /** Disable the ADC channel
    *
    * Disabling unused channels speeds up conversion in used channels. 
    * When disabled you can still call read, that will do a single conversion (actually two since the first one always returns 0 for unknown reason).
    * Then the function blocks until the value is read. This is handy when you sometimes needs a single conversion besides the automatic conversion
    */
    void disable( void );
    
    /** Returns the raw value
    *
    * @param return Unsigned integer with converted value
    */
    unsigned short read_u16( void );
    
    /** Returns the scaled value
    *
    * @param return Float with scaled converted value to 0.0-1.0
    */
    float read( void )
    {
        unsigned short value = read_u16();
        return (float)value * (1.0f/65535.0f);
    }
    
    /** An operator shorthand for read()
    */
    operator float() {
        return read();
    }

    
private:
    bool running;    
    char ADCnumber;
    volatile uint32_t *datareg;
};

#endif