A DIY 8-bit DAC and a visual python-based shape language library for drawing on an oscilloscope screen with an Arduino or Raspberry Pi.
Let's see how fast the Atmega328P can draw the circle generated by the following code:
python
f = open('circleCoord.txt',mode='w')
f.write('byte shapeX[] = {')
for i in range(0,360,2):
alpha=math.radians(i)
x = int(128*math.cos(alpha)+127.5)
if i != 358:
f.write(str(x)+', ')
else:
f.write(str(x)+'};\n')
f.write('byte shapeY[] = {')
for i in range(0,360,2):
alpha=math.radians(i)
y = int(128*math.sin(alpha)+127.5)
if i != 358:
f.write(str(y)+', ')
else:
f.write(str(y)+'};\n')
f.close()
| Time between crests | No. points in circle |
|---|---|
| 68 ms | 180 |
| 34 ms | 90 |
| 17 ms | 45 |
Bang, linear as hell! 2.65 points drawn/ms. That is BAD.
Now I'll try optimizing the code:
| Time between crests | No. points in circle |
|---|---|
| 130 ms | 300 |
| 65 ms | 180 |
| 32.5 ms | 90 |
| 16 ms | 45 |
Goddamn, it got worse! I will need do do something better. Let's try the bitRead() function directly on the coordinate integer
| Time between crests | No. points in circle |
|---|---|
| 46 ms | 300 |
| 24 ms | 200 |
| 23 ms | 180 |
| 16 ms | 100 |
| 11.5 ms | 90 |
| 5.8 ms | 45 |
Perfect! Now I can draw 180 points without obvious jitter.
Migrating the whole shebang to Raspberry Pi2
Using the RPi.GPIO library (0.6.2) I can shift a Point in 250-275 µs, which gives ≈ 160-170 points on screen without jitter. Shifting to the WiringPi C library (2.29) and the wiringPi-Python wrapper, I can shift a point in 450 µs, with ≈100 jitter-free on-screen points, by using the same type of for loop I was using with RPi.GPIO.
However, if I use the built in wiringpi.shiftOut() function, which directly shifts out 8-bit integer values to the shift registers, then I can shift a Point in 50 µs, for a whopping 840 simultaneous smooth-as-velvet on-screen points!!!
I wanted to test if there was any difference in which board setup I was using, but I couldn't get the physical pin numbering setup to work for some reason, so I just stuck with the Broadcom GPIO channel numbers.