Week 4: Embedded Programming

Group Assignment (link): compare workflows for available embedded architectures

Individual Assignment: 1) browse through the data sheet for a microcontroller; 2) write and test a program for an embedded system using a microcontroller to interact and communicate; 3) extra credit: try different languages &/or development environments.

RP2040 DataSheet

The RP2040 Datasheet gives full information about the RP2040 Microcontroller. Find it here.

Here's information about the name "RP2040"

Here's a detailed overview of the chip:

Many things here I don't know what they are, for example:

• SWD - communication protocol for bootloader etc
• QSPI - communication protocol for flash devices

Here's the pinout of the RP2040. Note that specific development boards can use more or less of the pins.

The Xiao-RP2040 has fewer GPIO than the Raspberry Pi Pico, though they both use the same Microcontroller!

In summary the chip, the microcontroller is amazing. So much packed into such small size. I'm learning a lot.

Some additional questions worth asking:

• What's the chip? Answer: RP2040 has a dual M0+ processor cores.
• what speed? Answer: the ARM cortex microprocessor has speed 133 MHz
• What flash / SRAM memory? Answer: External flash memory and 264kB of embedded SRAM in 6 banks
• How many GPIO pins? How many analog pins? Answer: Total 30 GPIO pins, 4 Analog pins
• What communication protocols? Answer: Yes, many.
• Does it have an ADC? Answer: Yes it has an analog to digital converter.

Programming SEED RP2040 with Thonny:

Thonny is a Python IDE. Search for, download and install that is easy at: thonny.org. As a group in Kochi we read and followed this documentation, which was extremely helpful. We made some simple programs like the following, mainly using the 3 little built-in LEDs.

      
from time import sleep
from machine import Pin

Blue=Pin(25,Pin.OUT)
Red=Pin(17,Pin.OUT)
Green=Pin(16,Pin.OUT)
Colours=[Blue,Red,Green]

def allblink():
    for colour in Colours:
        colour.low()
        sleep(0.3)
        colour.high()
        sleep(0.3)

while True:
    allblink()
      
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There were some difficulties, most important to note is that the MicroPython interpreter needs to be installed on the RP2040. You will need to do it again if the same microcontroller is used with Arduino code, since it compiles and sends there hex code.

To do this, navigate to "Run"->"Configure interpreter" and you'll see the following dialogue. You may need to hold the boot button on the board for your computer to recognize the device, but after it does, you'll select "Pico / Pico H" from the dropdown and just say "Okay".

Programming simulated Arduino (WOKWI):

Wokwi is a free and easy to use online simulator for microcontrollers. After the microcontroller is chosen, we can see here the interface with the programming canvas on left and simulated hardware set-up to the right.

There are buttons to start the simulation as well as to add components. Wiring is easily accomplished by simply clicking a pin on the microcontroller or on the component and just dragging it to the endpoint.

Here is a simple program, goal being to understand "INPUT_PULLUP" and its importance. Basically pullup is a directive to internally connect the pin to power through a resistor. The appropriate value of resistor can be found using Ohm's law, keeping in mind that the voltage drop across different color LEDs will vary.

Choosing resistor value: to choose the resistor value, we use Ohm's law. The red LED such as we use has a smaller forward voltage, around 2 V. This means that a smaller voltage is required to turn it on, and for this reason we'll need a larger resistor value to "eat up" that voltage. Using V=IR with V at a value of 3 and I = 20 mA, we calculate a required resistance value of 150 Ohm. We round up and use a 220 Ohm resistor in these examples.

In this follow-up program, the LED is toggled by the 2nd button, but it's not possible by the first button because it is not "pulled-up" and hence unstable.

      
void setup() {
Serial.begin(115200);
pinMode(A2, INPUT);
pinMode(A3, INPUT_PULLUP);

pinMode(13, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  int buttonState = digitalRead(A2);
  int button2State = digitalRead(A3);
  // print out the state of the button:
  Serial.print(buttonState);
  Serial.print("---");
  Serial.println(button2State);

  if (button2State==HIGH){
    digitalWrite(13,HIGH);
    }
  else{
    digitalWrite(13, LOW);
  }

  delay(1);  // delay in between reads for stability
}
      
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Eventually as a group in Kochi we abandoned the Arduino simulator as it too often failed to run.

Raspberry Pi PICO Simulations in WOKWI

Example 1 - LED with Switch:

Here's the simple code. If you flip the switch the light goes out:

      
import time
from machine import Pin
time.sleep(1) # Wait for USB to become ready

led=Pin(1,Pin.OUT)
switch = Pin(13, Pin.IN)

while True:
    if switch.value()==1:
        led.high()
        print('On')
    if switch.value()==0:
        led.low()
        print('Off')
    time.sleep(0.2)
      
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Example 2 - Ultrasonic:

We start by importing libraries, define pins, and wire the ultrasonic to the board. Just press 'R' to rotate the board.

Now we import and test a function for the ultrasonic we found from Tom's Hardware. It works!

Example 3 - OLED:

We continue with the Ultrasonic and connect an OLED, including a library "ssd1306.py" which we copied from another OLED program on Wokwi. From there we additionally must include a function for the communication protocol which is difficult to understand.

We add lines to the Ultra function to print to the OLED. We also define SDA/SCL pins. And it works alhamdulillah!

Here's the full code for this project:

      
from machine import Pin, I2C
from ssd1306 import SSD1306_I2C
import framebuf, sys
import utime

pix_res_x = 128
pix_res_y = 64

trigger = Pin(3, Pin.OUT)
echo = Pin(2, Pin.IN)

def init_i2c(scl_pin, sda_pin):
    # Initialize I2C device
    i2c_dev = I2C(1, scl=Pin(scl_pin), sda=Pin(sda_pin), freq=200000)
    i2c_addr = [hex(ii) for ii in i2c_dev.scan()]
    
    if not i2c_addr:
        print('No I2C Display Found')
        sys.exit()
    else:
        print("I2C Address      : {}".format(i2c_addr[0]))
        print("I2C Configuration: {}".format(i2c_dev))
    
    return i2c_dev

def ultra():
    trigger.low()
    utime.sleep_us(2)
    trigger.high()
    utime.sleep_us(5)
    trigger.low()
    while echo.value() == 0:
        signaloff = utime.ticks_us()
    while echo.value() == 1:
        signalon = utime.ticks_us()
    timepassed = signalon - signaloff
    distance = (timepassed * 0.0343) / 2
    print("The distance from object is ",distance,"cm")
        # Clear the specific line by drawing a filled black rectangle
    oled.fill_rect(5, 40, oled.width - 5, 8, 0)
    oled.text("distance:", 5, 30)
    oled.text(str(distance) + " cm", 5, 40)
    oled.show()


i2c_dev = init_i2c(scl_pin=27, sda_pin=26)
oled = SSD1306_I2C(pix_res_x, pix_res_y, i2c_dev)

while True:
   ultra()
   utime.sleep(1)
      
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Example 4 - Stepper Motor:

We wire up the Stepper Motor, however our example code is only in Arduino, so we must research how it can work in Micropython.

With some difficulty, research and help from perplexity.ai, we got the code working just for the single step motor to work.

Example 5 - All Together:

Final step is to bring this separate code for the stepper motor into our main program with the ultrasonic and OLED screen:

Everything seemed to be fine, but the stepper motor didn't move! Turns out we forgot the reset-sleep connection on the driver.

After we fixed that connection on the motor driver, everything worked fine. Here's a diagram of the driver sourced from here.

When I prepared to make a video, there were a couple imperfections, namely that the motor didn't reverse and part of the OLED display did not clear, but I was able to fix these coding issues and then make a screen recording:

Full code for the end of week project is here:

from machine import Pin, I2C
from ssd1306 import SSD1306_I2C
import framebuf, sys
import utime

utime.sleep(0.1) # Wait for USB to become ready
print("Povam!")

direction = Pin(0, Pin.OUT)
step = Pin(1, Pin.OUT)
step.value(0)

trigger = Pin(3, Pin.OUT)
echo = Pin(2, Pin.IN)

pix_res_x = 128
pix_res_y = 64

def init_i2c(scl_pin, sda_pin):
    # Initialize I2C device
    i2c_dev = I2C(1, scl=Pin(scl_pin), sda=Pin(sda_pin), freq=200000)
    i2c_addr = [hex(ii) for ii in i2c_dev.scan()]
    
    if not i2c_addr:
        print('No I2C Display Found')
        sys.exit()
    else:
        print("I2C Address      : {}".format(i2c_addr[0]))
        print("I2C Configuration: {}".format(i2c_dev))
    
    return i2c_dev

def move_motor(steps, clockwise, delay):
    direction.value(clockwise)  # Set direction: HIGH for CW, LOW for CCW
    for d in range(steps):
        step.value(1)  
        utime.sleep(delay)  
        step.value(0)  
        utime.sleep(delay)  

def ultra():
    trigger.low()
    utime.sleep_us(2)
    trigger.high()
    utime.sleep_us(5)
    trigger.low()
    while echo.value() == 0:
        signaloff = utime.ticks_us()
    while echo.value() == 1:
        signalon = utime.ticks_us()
    timepassed = signalon - signaloff
    distance = (timepassed * 0.0343) / 2
    print("The distance from object is ",distance,"cm")
    if (130
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