The display uses shift registers, those are all the chips on the back of the screen. This requires the Pi to do a lot of work bit-banging the pixels onto the screen. You can read more about how this display works here.
As an experiment, I decided to see how practical it would be to try and drive the display from Python code, rather than using the much faster C code.
I had fairly limited success in this. The display refreshes with a bit of flicker, is rather uneven in light intensity and dimmer than the C code. So if anyone can improve the code and make it work better, we would love to hear from you and be happy to update this section with due credit.
As an experiment, I decided to see how practical it would be to try and drive the display from Python code, rather than using the much faster C code.
I had fairly limited success in this. The display refreshes with a bit of flicker, is rather uneven in light intensity and dimmer than the C code. So if anyone can improve the code and make it work better, we would love to hear from you and be happy to update this section with due credit.
The code is relatively compact. It is crude, doing nothing in the way of PWM and can therefore only display eight colors.
If you want to give it a go, the wiring is exactly the same as before, and you will just need to paste the following code into a file called test_display.py
import RPi.GPIO as GPIO
import time
delay = 0.000001
GPIO.setmode(GPIO.BCM)
red1_pin = 17
green1_pin = 18
blue1_pin = 22
red2_pin = 23
green2_pin = 24
blue2_pin = 25
clock_pin = 3
a_pin = 7
b_pin = 8
c_pin = 9
latch_pin = 4
oe_pin = 2
GPIO.setup(red1_pin, GPIO.OUT)
GPIO.setup(green1_pin, GPIO.OUT)
GPIO.setup(blue1_pin, GPIO.OUT)
GPIO.setup(red2_pin, GPIO.OUT)
GPIO.setup(green2_pin, GPIO.OUT)
GPIO.setup(blue2_pin, GPIO.OUT)
GPIO.setup(clock_pin, GPIO.OUT)
GPIO.setup(a_pin, GPIO.OUT)
GPIO.setup(b_pin, GPIO.OUT)
GPIO.setup(c_pin, GPIO.OUT)
GPIO.setup(latch_pin, GPIO.OUT)
GPIO.setup(oe_pin, GPIO.OUT)
screen = [[0 for x in xrange(32)] for x in xrange(16)]
def clock():
GPIO.output(clock_pin, 1)
GPIO.output(clock_pin, 0)
def latch():
GPIO.output(latch_pin, 1)
GPIO.output(latch_pin, 0)
def bits_from_int(x):
a_bit = x & 1
b_bit = x & 2
c_bit = x & 4
return (a_bit, b_bit, c_bit)
def set_row(row):
#time.sleep(delay)
a_bit, b_bit, c_bit = bits_from_int(row)
GPIO.output(a_pin, a_bit)
GPIO.output(b_pin, b_bit)
GPIO.output(c_pin, c_bit)
#time.sleep(delay)
def set_color_top(color):
#time.sleep(delay)
red, green, blue = bits_from_int(color)
GPIO.output(red1_pin, red)
GPIO.output(green1_pin, green)
GPIO.output(blue1_pin, blue)
#time.sleep(delay)
def set_color_bottom(color):
#time.sleep(delay)
red, green, blue = bits_from_int(color)
GPIO.output(red2_pin, red)
GPIO.output(green2_pin, green)
GPIO.output(blue2_pin, blue)
#time.sleep(delay)
def refresh():
for row in range(8):
GPIO.output(oe_pin, 1)
set_color_top(0)
set_row(row)
#time.sleep(delay)
for col in range(32):
set_color_top(screen[row][col])
set_color_bottom(screen[row+8][col])
clock()
#GPIO.output(oe_pin, 0)
latch()
GPIO.output(oe_pin, 0)
time.sleep(delay)
def fill_rectangle(x1, y1, x2, y2, color):
for x in range(x1, x2):
for y in range(y1, y2):
screen[y][x] = color
def set_pixel(x, y, color):
screen[y][x] = color
fill_rectangle(0, 0, 12, 12, 1)
fill_rectangle(20, 4, 30, 15, 2)
fill_rectangle(15, 0, 19, 7, 7)
while True:
refresh()
You can run the program using the command:
$ sudo python test_display.py
Looking at the end of the file, you can see the "fill_rectangle" and "set_pixel" functions that you can use to draw on the screen. The final argument of both of these functions is the color that should be a number between 0 and 7.
The display refreshing should really take place in a separate thread of execution, but as I said, this is more of an experiment than anything else.
The display refreshing should really take place in a separate thread of execution, but as I said, this is more of an experiment than anything else.
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