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.

Committer:
Sissors
Date:
Thu May 09 17:46:32 2013 +0000
Revision:
0:c2a7b899e6c7
Child:
2:9b61d0792927
v0.1
;

Who changed what in which revision?

UserRevisionLine numberNew contents of line
Sissors 0:c2a7b899e6c7 1 #ifndef FASTANALOGIN_H
Sissors 0:c2a7b899e6c7 2 #define FASTANALOGIN_H
Sissors 0:c2a7b899e6c7 3
Sissors 0:c2a7b899e6c7 4 /*
Sissors 0:c2a7b899e6c7 5 * Includes
Sissors 0:c2a7b899e6c7 6 */
Sissors 0:c2a7b899e6c7 7 #include "mbed.h"
Sissors 0:c2a7b899e6c7 8 #include "pinmap.h"
Sissors 0:c2a7b899e6c7 9
Sissors 0:c2a7b899e6c7 10 #ifndef TARGET_LPC1768
Sissors 0:c2a7b899e6c7 11 #error "Target not supported"
Sissors 0:c2a7b899e6c7 12 #endif
Sissors 0:c2a7b899e6c7 13
Sissors 0:c2a7b899e6c7 14 /** A class that is similar to AnalogIn, only faster
Sissors 0:c2a7b899e6c7 15 *
Sissors 0:c2a7b899e6c7 16 * AnalogIn does a single conversion when you read a value (actually several conversions and it takes the median of that).
Sissors 0:c2a7b899e6c7 17 * This library has all used ADC channels running automatically in the background, and if you read a value it will immediatly return the last converted value.
Sissors 0:c2a7b899e6c7 18 *
Sissors 0:c2a7b899e6c7 19 * You can use several FastAnalogIn objects per ADC pin, and several ADC pins at the same time. Using more ADC pins will decrease the conversion rate.
Sissors 0:c2a7b899e6c7 20 * If you need to generally convert one pin very fast, but sometimes also need to do AD conversions on another pin, you can disable the ADC channel and still read its value.
Sissors 0:c2a7b899e6c7 21 * Only now it will block until conversion is complete, so more like the regular AnalogIn library. Of course you can also disable an ADC channel and enable it later.
Sissors 0:c2a7b899e6c7 22 *
Sissors 0:c2a7b899e6c7 23 * It does not play nicely with regular AnalogIn objects, so use only this library or only AnalogIn. Also currently only LPC1768 is supported.
Sissors 0:c2a7b899e6c7 24 */
Sissors 0:c2a7b899e6c7 25 class FastAnalogIn {
Sissors 0:c2a7b899e6c7 26
Sissors 0:c2a7b899e6c7 27 public:
Sissors 0:c2a7b899e6c7 28 /** Create a FastAnalogIn, connected to the specified pin
Sissors 0:c2a7b899e6c7 29 *
Sissors 0:c2a7b899e6c7 30 * @param pin AnalogIn pin to connect to
Sissors 0:c2a7b899e6c7 31 * @param enabled Enable the ADC channel (default = true)
Sissors 0:c2a7b899e6c7 32 */
Sissors 0:c2a7b899e6c7 33 FastAnalogIn( PinName pin, bool enabled = true );
Sissors 0:c2a7b899e6c7 34
Sissors 0:c2a7b899e6c7 35 ~FastAnalogIn( void );
Sissors 0:c2a7b899e6c7 36
Sissors 0:c2a7b899e6c7 37 /** Enable the ADC channel
Sissors 0:c2a7b899e6c7 38 *
Sissors 0:c2a7b899e6c7 39 * @param enabled Bool that is true for enable, false is equivalent to calling disable
Sissors 0:c2a7b899e6c7 40 */
Sissors 0:c2a7b899e6c7 41 void enable(bool enabled = true);
Sissors 0:c2a7b899e6c7 42
Sissors 0:c2a7b899e6c7 43 /** Disable the ADC channel
Sissors 0:c2a7b899e6c7 44 *
Sissors 0:c2a7b899e6c7 45 * Disabling unused channels speeds up conversion in used channels.
Sissors 0:c2a7b899e6c7 46 * 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).
Sissors 0:c2a7b899e6c7 47 * Then the function blocks until the value is read. This is handy when you sometimes needs a single conversion besides the automatic conversion
Sissors 0:c2a7b899e6c7 48 */
Sissors 0:c2a7b899e6c7 49 void disable( void );
Sissors 0:c2a7b899e6c7 50
Sissors 0:c2a7b899e6c7 51 /** Returns the raw value
Sissors 0:c2a7b899e6c7 52 *
Sissors 0:c2a7b899e6c7 53 * @param return Unsigned integer with converted value
Sissors 0:c2a7b899e6c7 54 */
Sissors 0:c2a7b899e6c7 55 unsigned short read_u16( void );
Sissors 0:c2a7b899e6c7 56
Sissors 0:c2a7b899e6c7 57 /** Returns the scaled value
Sissors 0:c2a7b899e6c7 58 *
Sissors 0:c2a7b899e6c7 59 * @param return Float with scaled converted value to 0.0-1.0
Sissors 0:c2a7b899e6c7 60 */
Sissors 0:c2a7b899e6c7 61 float read( void );
Sissors 0:c2a7b899e6c7 62
Sissors 0:c2a7b899e6c7 63 /** An operator shorthand for read()
Sissors 0:c2a7b899e6c7 64 */
Sissors 0:c2a7b899e6c7 65 operator float() {
Sissors 0:c2a7b899e6c7 66 return read();
Sissors 0:c2a7b899e6c7 67 }
Sissors 0:c2a7b899e6c7 68
Sissors 0:c2a7b899e6c7 69
Sissors 0:c2a7b899e6c7 70 private:
Sissors 0:c2a7b899e6c7 71 static const PinMap PinMap_ADC[9];
Sissors 0:c2a7b899e6c7 72 static int channel_usage[8];
Sissors 0:c2a7b899e6c7 73
Sissors 0:c2a7b899e6c7 74 bool running;
Sissors 0:c2a7b899e6c7 75 char ADCnumber;
Sissors 0:c2a7b899e6c7 76 uint32_t *datareg;
Sissors 0:c2a7b899e6c7 77
Sissors 0:c2a7b899e6c7 78
Sissors 0:c2a7b899e6c7 79
Sissors 0:c2a7b899e6c7 80
Sissors 0:c2a7b899e6c7 81 };
Sissors 0:c2a7b899e6c7 82
Sissors 0:c2a7b899e6c7 83 #endif