Arduino-AD-Multichannel-Multitasking-Interrupt

(C) 2019 Dipl. Phys. Helmut Weber

Reading AD-Valures, which are generated by interrupts
ATMega328p: UNO, NANO, ...

Sketch uses 3872 bytes (12%) of program storage space. Maximum is 30720 bytes. Global variables use 388 bytes (18%) of dynamic memory, leaving 1660 bytes for local variables. Maximum is 2048 bytes.

Introduction

Realtime Operating systems are the prefered tool for most measurements.
ChibiOS and derivatives are running on the Arduino UNO.
But sometimes a TickTime of 1 ms is too long.

The goal is: Measuring AD values at 3 channels (or more) as fast as possible and print them ready for Arduino-Plotter. 3 Channels mean: Free running is not possible because we have to change the channels.

As an example a HEART-BEAT sensor is used.The data line is connected to A0, A1, A2 - but 3 different
AD-Sources could be used.

The 3 lines are filtered because they are noisy. Different filter factors are used.

Interrupts

Here I show another approach using Interrupts. Three curves are red from the AD-converter using AD-Interrupt-Conversion-Ready
Up to 6000 conversions per second are done.
The values are filtered and may be displayed using "Serial Plotter"

Multitasking

Besides "loop" 2 tasks are running in the backgrund"
One of them every 1 ms, the other more often but with more jitter !

Filter

AD-Values for CHNUM channels are red using AD-Ready-Interrupts. One Channel after the other is red starting with channel 0 again.

This is done in the background without any intervention of the LOOP

The values are averaged for IRQ_SAMPLES samples

This is the code for averaging: > avv=(float)analogVal;
> av[Channel] = (Alpha[Channel]*oldVal[Channel]) + ((1-Alpha[Channel])*avv);
> oldVal[Channel]=av[Channel];

The sense is to get most samples possible, build an average over IRQ_SAMPLES value and set the IrqReadyFlag. Then the Measurement is stopped and the "Busy" Flag is set.

LOOP or any other function has to read (and print/ plot) the values.

After that the next points will be generated in the background again with:

IrqReadyFlag = 0;
Busy = false;
ADCSRA |= B01000000; // Start next conversion

Example-HEART-BEAT

Here is an example of a HEART-BEAT-Sensor.

Blue - original

Green - soft filtered

Red - hard filtered<br

The channels got an offset for better display.

Filtereing implies a phase shift !

Timing

We get about 5500 (filtered) samples per second.

• with 10 IRQ_SAMPLES there are 200 Time Points per second (180 with Serial.print)

Enough to show the details of HEART-BEAT

3 Channels with 10 samples = 30 samples in 5,4 ms for a Time Point.

There is plenty of room to do other things in LOOP !

@author Helmut Weber