I’m trying to use MCP3002 for the first time, and can’t seem to get the basics to work
I’ve been following this guide and am pretty certain that all my connections are right (updated photo below).
Running the program below, I can’t make sense of the figures i’m getting – using the RPis 3.3V and with this as the potentiometer, I get either 1024 or 0 values irrespective of the position of the potentiometer.
I have added a led to check that the potentiometer works. I’ve tested all the wires to make sure the crimping is ok.
What am I getting wrong and/or how can I try and troubleshoot to look for any potential issues?
# botbook_mcp3002.py - read analog values from mcp3002 # (c) BotBook.com - Karvinen, Karvinen, Valtokari # Installing spidev: # sudo apt-get update # sudo apt-get -y install git python-dev # git clone https://github.com/doceme/py-spidev.git # cd py-spidev/ # sudo python setup.py install import spidev # installation help in botbook_mcp3002.py comments import time def readAnalog(device = 0,channel = 0): assert device in (1, 0) assert channel in (1, 0) #open spi spi = spidev.SpiDev() #spi.open(0, device) spi.open(0, device) """ Protocol start bit (S), sql/diff (D), odd/sign (C), MSBF (M) Use leading zero for more stable clock cycle 0000 000S DCM0 0000 0000 0000 Sending 3 8bit packages so xpi.xfer2 will return the same amount. start bit = 1 sql/diff = 1 SINGLE ENDED MODE (2 channel mode) odd/sign = channel 0/1 MSBF = 0 """ command = [1, (2 + channel) << 6, 0] #2 + channel shifted 6 to left #10 or 11 << 6 = 1000 0000 or 1100 0000 reply = spi.xfer2(command) """ Parse right bits from 24 bit package (3*8bit) We need only data from last 2 bytes. And there we can discard last two bits to get 10 bit value as MCP3002 resolution is 10bits Discard reply byte and start from reply where our data starts """ value = reply & 31 #31 = 0001 1111 with & operation makes sure that we have all data from XXXX DDDD and nothing more. 0001 is for signed in next operation. value = value << 6 #Move to left to make room for next piece of data. #000D DDDD << 6 = 0DDD DD00 0000 #Now we get the last of data from reply value = value + (reply >> 2) #Here we discard last to bits #DDDD DDXXX >> 2 = 00DD DDDD #0DDD DD00 0000 + 00DD DDDD = 0DDD DDDD DDDD spi.close() return value def main(): #read channel 0 on device 0 value = readAnalog(0, 0) print(value) #time.sleep(10) if __name__ == "__main__": main()