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How to take inputs faster using Raspberry Pi

John Manuel

Jun 7, 2018
20
Joined
Jun 7, 2018
Messages
20
I am trying to detect different frequency signals using a Raspberry Pi and an ADC converter (PCF8591). My initial guess was the highest frequency I could detect would be limited by the speed of the I2C bus (which is about 400000 bauds). But when I run the setup, I could only get sampling speeds of around 10000 samples/sec. This value lies very close to the time taken to run an empty for loop in python (which is about 10^(-5) seconds which is equivalent to 10^(5) samples/sec).

I wanted to reach higher frequency domain (around 10 MHz) and using a better ADC won't be of much help since the limitation is set by the program itself. I wanted to know if my conclusion is correct or not. I have a feeling I am very wrong and doing something stupid. Any guidance will be much appreciated. Below is the python code I am using for signal detection.

Code:
#!/usr/bin/python
# -*- coding:utf-8 -*-
import smbus
import time
import matplotlib.pyplot as plt

start = time.time()

address = 0x48
A0 = 0x40
A1 = 0x41
A2 = 0x42
A3 = 0x43
bus = smbus.SMBus(1)
bus.write_byte_data(address,0,0b00010000)#control byte to tell ADC to act as a differential input

voltage_value = [ ]
time_value = [ ]

try:
    while True:
        bus.write_byte(address,A0)
        value = bus.read_byte(address)
        voltage_value.append(value)
        time_value.append(time.time()-start)
        #print("AOUT:%1.3f  " %(value*3.3/255))
        #print(value)
        #time.sleep(0.1)
except KeyboardInterrupt:
    voltage_value = [x*3.3/255 for x in voltage_value]
    plt.plot(time_value,voltage_value)
    plt.ylabel('Voltage')
    plt.xlabel('Time')
    plt.show()

    with open('output.txt', 'w') as f:
        for v,t in zip(voltage_value,time_value):
            f.write(str(v)+' '+str(t)+'\n')
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,336
Joined
Nov 17, 2011
Messages
13,336
Python code is interpreted and much too slow to detect a 10 MHz signal. You'l also have to take into account that the Raspi runs Linux which is not a real time operating system and as such not suitable for such measurements. The 10000 samples/s are probably as much as you can get. Using C instead of Python you may crank a few more samples out of the Raspi, but not 10 MHz.
To measure in that range you'll need additional circuitry. Try "frequency measurement raspberry" in your favorite search engine for some suggestions.
 
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