ledpwm.py File Reference

Simple program using button press to cycle through three different LED patterns. More...

Functions
def	ledpwm.onButtonPressFCN (IRQ_src)
	Gives button ability to change a variable. More...
def	ledpwm.square (tdiff)
	Square wave pattern function. More...
def	ledpwm.sine (tdiff)
	Sine wave pattern function. More...
def	ledpwm.sawtooth (tdiff)
	Sawtooth wave pattern function. More...

Variables
bool	ledpwm.button_press = False
	Boolean variable to define if the button is pressed. More...
int	ledpwm.state = 0
	Boolean variable to define if the button is pressed. More...
	ledpwm.t0 = utime.ticks_ms()
	Starts timer zero point. More...
	ledpwm.pinC13 = pyb.Pin (pyb.Pin.cpu.C13)
	Pin variable for blue user button. More...
	ledpwm.pinA5 = pyb.Pin (pyb.Pin.cpu.A5)
	Pin variable for LED. More...
	ledpwm.tim2 = pyb.Timer(2, freq = 20000)
	Pin variable for pyb timer. More...
	ledpwm.t2ch1 = tim2.channel(1, pyb.Timer.PWM, pin=pinA5)
	Channel variable for pyb timer.
	ledpwm.ButtonInt
	Callback interrupt variable. More...
float	ledpwm.td = 0.001*utime.ticks_diff(utime.ticks_ms(),t0)
	Time elapsed since last button press. More...

Detailed Description

Simple program using button press to cycle through three different LED patterns.

In our first lab using hardware, the goal was to learn about interupt callbacks, pulse width modulation, and some other key basic concepts for working with the Nucleo board. This program allows the user to cycle through three different patterns (square, sine, and sawtooth) simply by presing the blue button on the Nucleo board. Included below are more details regarding source code, the general finite state machine implemented, and user defined functions.
See source code here: https://bitbucket.org/ryanmclaug/me_305/src/master/Lab0x02/ledpwm.py

See demo video here: https://youtu.be/7zcb3uxbQ7k

Author

Ryan McLaughlin

Date

Originally created on 01/19/21
Last modified on 01/31/21

Function Documentation

onButtonPressFCN()

def ledpwm.onButtonPressFCN

(

IRQ_src

)

Gives button ability to change a variable.

In order to change the state of the finite state machine, an interrupt callback is used in the form of the blue user button on the Nucleo board. When the callback function is called from the main script, the global variable button_press becomes True, and this change triggers the main script to update the state.

Parameters

IRQ_src

is

sawtooth()

def ledpwm.sawtooth

(

tdiff

)

Sawtooth wave pattern function.

The final waveform in the three pattern sequence is a sawtooth (or ramping) waveform. For this wave, the formula,

duty = 100 * ( t%1 )

is employed. The modulus operator reduces the input time to a vavlue between 0 and 1 (discluding 1), which can then be multiplied by 100 to get a correct brightness value.

Parameters

tdiff

Calculated in the main script, this is the time elapsed since the button was last pressed. The same input parameter is used for each of the three functions, and is calculated using a combination of utime.ticks_ms() and utime.ticks_diff().

sine()

def ledpwm.sine

(

tdiff

)

Sine wave pattern function.

The second waveform shown on the LED is a shifted sine wave. For this wave, the formula,

duty = 50 * ( 1 + sin( .2π*t ) )

is employed. Since a sine wave is inherently continuous in nature, operators are not needed in this function to modify the input time value. This wave has an amplitude of 50 (to oscillate between 0% and 100%) and is shifted upward to begin at 50% brightness.

Parameters

tdiff

See the above function sine() for a full description of tdiff

square()

def ledpwm.square

(

tdiff

)

Square wave pattern function.

The first of three pulsing waveforms is the classic square wave. For this wave, the formula,

duty = 100 * ( (t%1) <= 0.5 )

is employed. The modulus operator reduces the input to a value between 0 and 1 (discluding 1), followed by a comparision with 0.5. With this, the LED will switch between on and off every half second.

Parameters

tdiff

See the above function sine() for a full description of tdiff

Variable Documentation

button_press

bool ledpwm.button_press = False

Boolean variable to define if the button is pressed.

When the button has been pressed, the callback function is implemented to change button_press to True. This variable therefore defines when to switch states.

ButtonInt

ledpwm.ButtonInt

Initial value:

=  pyb.ExtInt(pinC13, mode=pyb.ExtInt.IRQ_FALLING,                 # define interrupt
                        pull=pyb.Pin.PULL_NONE, callback=onButtonPressFCN)

Callback interrupt variable.

When the blue button is pressed, it triggers the callback variable, running the callback function onButtonPressFCN.

pinA5

ledpwm.pinA5 = pyb.Pin (pyb.Pin.cpu.A5)

Pin variable for LED.

Similar to pinC13, but for the output LED.

pinC13

ledpwm.pinC13 = pyb.Pin (pyb.Pin.cpu.C13)

Pin variable for blue user button.

The blue button on the board serves as the main user input interface, and therefore needs a pin assignment.

state

int ledpwm.state = 0

Boolean variable to define if the button is pressed.

When the button has been pressed, the callback function is implemented to change button_press to True. This variable therefore defines when to switch states.

t0

ledpwm.t0 = utime.ticks_ms()

Starts timer zero point.

To later calculate time elapsed since a state has been entered, a timer needs to start running.

td

float ledpwm.td = 0.001*utime.ticks_diff(utime.ticks_ms(),t0)

Time elapsed since last button press.

Since each pattern function is based on time, td is passed through to the correct function, and is used to calculate what brightness level the LED should be set to.

tim2

ledpwm.tim2 = pyb.Timer(2, freq = 20000)

Pin variable for pyb timer.

To use pulse width modulation of the LED, we need a timer.