FunctionGenerator

Arduino library to generate (numeric) wave forms for a DAC.

Description

This library presents a class for a function generator in software. It is typical used to control one or more DAC's. To maximize signal quality one has to apply all (or most) processor power to calculate new values over and over again to get enough resolution. In practice the generator is useful for low frequencies, 0.01 - 25 Hz, depending on waveform and processor and number of DAC's. (see indication below).

Note: this class generates float values, performance wise this can be optimized, to achieve higher speeds at cost of accuracy / precision.

Performance

Indication of what performance can be expected (based upon 0.2.1 version).
Note that the values need to be transported to a DAC or serial port too.
Numbers based on performance example, for one single signal.

Processor	Clock	Waveform	usec/call	max freq	max values/period
Arduino UNO	16 MHz	sawtooth	62	60 Hz	268
Arduino UNO	16 MHz	triangle	74	50 Hz	270
Arduino UNO	16 MHz	square	53	1000 Hz	19
Arduino UNO	16 MHz	sinus	164	25 Hz	152
Arduino UNO	16 MHz	stair	81	50 Hz	246
Arduino UNO	16 MHz	random	37	1000 Hz	27
ESP32	240 MHz	sawtooth	3.8	1000 Hz	263
ESP32	240 MHz	triangle	3.9	1000 Hz	256
ESP32	240 MHz	square	2.8	1000 Hz	357
ESP32	240 MHz	sinus	13.6	250 Hz	294
ESP32	240 MHz	stair	4.8	800 Hz	260
ESP32	240 MHz	random	1.3	1000 Hz	769

• Assumption minimal around 250 samples per period to get a smooth signal. If a rougher signal is OK, higher frequencies are possible. For square() and random() less samples per period are often acceptable.
• ESP32 can do more calculations however 1000 Hz seems to be a nice upper limit for a software based function generator.
• If one needs to control multiple DAC's one should divide the numbers and round down to get an estimate.

Note: hardware function generator https://github.com/RobTillaart/AD985X

Note: 0.2.5 due to duty cycle the triangle and square have become slightly slower.

Processor	Clock	Waveform	usec/call	max freq
Arduino UNO	16 MHz	triangle	84	50 Hz
Arduino UNO	16 MHz	square	57	1000 Hz
Arduino UNO	16 MHz	random_DC	68	500 Hz

Accuracy

If the time parameter t grows large, the internal math may have rounding problems after some time. This can and will affect the quality of the output.

Needs further investigations.

Interface

#include "functionGenerator.h"

Constructor

• funcgen(float period = 1.0, float amplitude = 1.0, float phase = 0.0, float yShift = 0.0) All parameters (except duty cycle) can be set in the constructor but also later in configuration. Default dutyCycle is 50%.

Configuration

• void setPeriod(float period = 1.0) set the period of the wave in seconds.
• float getPeriod() returns the set period.
• void setFrequency(float frequency = 1.0) set the frequency of the wave in Hertz (1/s).
• float getFrequency() returns the set frequency in Hertz.
• void setAmplitude(float amplitude = 1.0) sets the amplitude of the wave. The range is from -amplitude to +amplitude. Setting the amplitude to 0 gives effectively a zero signal. Setting the amplitude to a negative value effectively inverts the signal.
• float getAmplitude() returns the set amplitude.
• void setPhase(float phase = 0.0) shifts the phase of the wave. Will only be noticeable when compared with other waves. Phase is also known as the X- or horizontal shift.
• float getPhase() returns the set phase.
• void setYShift(float yShift = 0.0) sets an Y-shift or vertical offset in amplitude. This allows to set e.g. the zero level.
• float getYShift() returns the set Y-shift.
• void setDutyCycle(float percentage = 100) sets the duty cycle of the signal. Experimental, not all waveforms have a duty cycle or interpret it differently, see below. Duty cycle must be between 0 and 100% and will be clipped otherwise.
• float getDutyCycle() returns the set duty cycle.
• void setRandomSeed(uint32_t a, uint32_t b = 314159265) sets the initial seeds for the (Marsaglia) random number generator. The first is mandatory, the second is optional.

Wave forms

The variable t == time in seconds.

• float sawtooth(float t, uint8_t mode = 0) mode 0 is default.
  • mode == 0 ==> sawtooth /|.
  • mode == 1 ==> sawtooth |\. Effectively equals inverting the amplitude.
• float triangle(float t) triangle form, duty cycle default 50%.
• float square(float t) square wave with duty cycle default 50%.
• float sinus(float t) sinus wave, has no duty cycle.
• float stair(float t, uint16_t steps = 8, uint8_t mode = 0) defaults to 8 steps up.
  • mode = 0 ==> steps up
  • mode = 1 ==> steps down. Effectively equals inverting the amplitude.
• float random() random noise generation.
• float line() constant voltage line. Height depends on the YShift and amplitude.
• float zero() constant zero.

The functions line() and zero() can be used to drive a constant voltage from a DAC and can be used to calibrate the generator / DAC combination.

Duty Cycle

Since 0.2.5 the library has experimental support for duty cycle. The meaning of duty cycle differs per wave form. Implementation may change in the future.

In first iteration the following behaviour is implemented:

• square() implements duty cycle in a well known way. At the start of the period the signal goes "HIGH". After duty cycle % of the period the signal goes LOW until the end of the period.
• triangle() function uses the duty cycle to shift the peak from the begin (0%) to middle (50%) to end (100%) of the period.
• random_DC() A duty cycle of 0% is no noise 100% is full amplitude noise with respect to previous value. Implemented as a weighed average between new and previous value. Made a separate function as handling the duty cycle slows performance substantial. Initial starts at zero and can be adjusted with YShift().

The other functions need to be investigated what duty cycle means. Current ideas that are NOT implemented:

• sawtooth() - move from /|. to /|__ so 0% is a spike, 100% = normal. Think of it as the halve of the triangle wave.
• sinus() move the two peaks like the triangle (adjusting steepness / freq)??
• stair() like sawtooth??
• line() has no period so does not make sense (yet).
• zero() has no period so does not make sense (yet).

Feedback and ideas are welcome.

Future

Must

• documentation
  • quality of signals - after substantial time t
  • max freq per wave form etc. Should this be in the library?

Should

• smart reseed needed for random().
• initialize random generator with compile time.

Could

• waves
  • trapezium wave (could merge square and triangle and sawtooth)
  • white noise, pink noise etc.
• RC function curve.
• external clock to synchronize two or more software function generators.
• stand-alone functions in separate .h
• check for synergy with https://github.com/RobTillaart/AD985X
• investigate performance.
  • algorithms for DAC specific gains e.g. 10-12-16 bit.
  • improve performance sin() lookup table.
  • add float variable for _perDC = _period * _dutyCycle
  • do we need freq4 ? not since DC.

Examples

• Amplitude modulation ?
• heartbeat curve?
• example ESP32 version as separate task.
• example with DAC. 8 12 16 bit.
• example with potentiometers for 4 parameters

Wont

• investigate duty cycle for waveforms
  • Derived class for the duty cycle variants? or functions!
  • float squareDC() performance (loss)
  • float triangleDC()
  • float sawtoothDC()
  • float sinusDC() duty-cycle for sinus what does it mean.
    • ==> move peaks, two half sinus with diff frequency
  • float stairDC()
• Bezier curve? (too complex)
• record a signal and play back ==> separate class

Support

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Thank you,