My Experiments with Raspberry Pi: Part 1

Today I am going to run a 7 segment display off of raspberry pi 4.

The code I ran

#!/usr/bin/env python
import RPi.GPIO as GPIO
import time

pins = [11,12,13,15,16,18,22,7]
dats = [0x3f,0x06,0x5b,0x4f,0x66,0x6d,0x7d,0x07,0x7f,0x6f,0x77,0x7c,0x39,0x5e,0x79,0x71,0x80]

def setup():
 GPIO.setmode(GPIO.BOARD)
 for pin in pins:
  GPIO.setup(pin, GPIO.OUT)   # Set pin mode as output
  GPIO.output(pin, GPIO.LOW)

def writeOneByte(val):
 GPIO.output(11, val & (0x01 << 0))
 GPIO.output(12, val & (0x01 << 1))
 GPIO.output(13, val & (0x01 << 2))
 GPIO.output(15, val & (0x01 << 3))
 GPIO.output(16, val & (0x01 << 4))
 GPIO.output(18, val & (0x01 << 5))
 GPIO.output(22, val & (0x01 << 6))
 GPIO.output(7,  val & (0x01 << 7))

def loop():
 while True:
  for dat in dats:
   writeOneByte(dat)
   time.sleep(0.5)

def destroy():
 for pin in pins:
  GPIO.output(pin, GPIO.LOW)
 GPIO.cleanup()             # Release resource

if __name__ == '__main__':     # Program start from here
 setup()
 try:
  loop()
 except KeyboardInterrupt:  # When 'Ctrl+C' is pressed, the child program destroy() will be executed.
  destroy()

Here is the thing, I just copied this code which was given to me. I don’t know what this means. Specifically, the dats stuff. So let me just run and see if this works. Here is the circuit I made.

Remark: the 220 $ \Omega $ resistor is connected to power in case that is not clear.

It works 😅 . Let’s carry on and celebrate with a few 📷 .

Now that I have calmed down a little bit, let me look at the mysterious bit of code. From what’s happening I know that the code switches the leds in the 7-segment display so that we loop through 0 to 9 and A to F and then 0 again. The display changes every 0.5 seconds.

Time for google 🔍 . Found it 🙌. Let me rewrite the explanation in my own way.

What Was that Code

The parts that I didn’t understand were 0x3f-like objects and the operation <<.

Let me first understand 0x3f. Upon googling I found this on the wikipedia page of 7-segment display. This is the hexadecimal code for 0b00111111 binary code.

When I look at the python documentation x << y is a bitwise operation. And what it does is it returns x with the bits shifted to the left by y units (and new bits on the right-hand-side are zeros). At the same link I saw that x & y is the bitwise and. Each bit of the output is 1 if the corresponding bit of x AND of y is 1, otherwise it’s 0. Let me look at a few examples.

>>> bin(0x01)
'0b1'
>>> bin(0x01 << 0)
'0b1'
>>> bin(0x01 << 1)
'0b10'
>>> bin(0x01 << 2)
'0b100'
>>> bin(0x06)
'0b110'
>>> bin(0x06 << 3)
'0b110000'

>>> bin(0b00111111 & 0b00000001)
'0b1'
>>> bin(0b00111111 & 0b00010001)
'0b10001'
>>> bin(0b10111111 & 0b10010001)
'0b10010001'

I think this makes these operations clear to me. In case of x<<y you add y number of zeros to the binary form of x. x & y looks at the binary form of x and y and performs the and operation on every bit. In case this isn’t clear yet let me write the case of 0b00111111 & 0b00000001 again:

0b00111111
0b00000001
    ||
0b00000001
# which is the same as 0b1

To understand what is happening I have to look at the circuit I have made. I notice that pin 11 (using the raspberry pi board convention) is connected to the pin ?? on the 7-segment display.

Now let’s look at this statement of the code.

def writeOneByte(val):
 GPIO.output(11, val & (0x01 << 0))
 GPIO.output(12, val & (0x01 << 1))
 GPIO.output(13, val & (0x01 << 2))
 GPIO.output(15, val & (0x01 << 3))
 GPIO.output(16, val & (0x01 << 4))
 GPIO.output(18, val & (0x01 << 5))
 GPIO.output(22, val & (0x01 << 6))
 GPIO.output(7,  val & (0x01 << 7))

If I also look at how the loop() function is running, at any moment, val is an element in the list dats. Let’s consider the first case of val = 0x3f. What will be the output of val & (0x01 << 0)? This is the same as 0x3f & (0x01 << 0). I am going to run it in python.

>>> bin(0x3f & (0x01 << 0))
'0b1'