在没有接口的情况下读取树莓中的模拟信号

Question

I need to read an analog signal in raspberry and for this purpose I bought an MCP3002 circuit. i plug it in with the correct connections and i have found sample codes over the internet but it doesn't work.

Do I need to have an interface or I can do the job without it? Do you have any ideas what can go wrong?

Do you have a simple code to read the analog input?

The code I used is the following:

#!/usr/bin/env python

# Written by Limor "Ladyada" Fried for Adafruit Industries, (c) 2015
# This code is released into the public domain

import time
import os
import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
DEBUG = 1

# read SPI data from MCP3008 chip, 8 possible adc's (0 thru 7)
def readadc(adcnum, clockpin, mosipin, misopin, cspin):
        if ((adcnum > 7) or (adcnum < 0)):
                return -1
        GPIO.output(cspin, True)

        GPIO.output(clockpin, False)  # start clock low
        GPIO.output(cspin, False)     # bring CS low

        commandout = adcnum
        commandout |= 0x18  # start bit + single-ended bit
        commandout <<= 3    # we only need to send 5 bits here
        for i in range(5):
                if (commandout & 0x80):
                        GPIO.output(mosipin, True)
                else:
                        GPIO.output(mosipin, False)
                commandout <<= 1
                GPIO.output(clockpin, True)
                GPIO.output(clockpin, False)

        adcout = 0
        # read in one empty bit, one null bit and 10 ADC bits
        for i in range(12):
                GPIO.output(clockpin, True)
                GPIO.output(clockpin, False)
                adcout <<= 1
                if (GPIO.input(misopin)):
                        adcout |= 0x1

        GPIO.output(cspin, True)

        adcout >>= 1       # first bit is 'null' so drop it
        return adcout

# change these as desired - they're the pins connected from the
# SPI port on the ADC to the Cobbler
SPICLK = 18
SPIMISO = 23
SPIMOSI = 24
SPICS = 25

# set up the SPI interface pins
GPIO.setup(SPIMOSI, GPIO.OUT)
GPIO.setup(SPIMISO, GPIO.IN)
GPIO.setup(SPICLK, GPIO.OUT)
GPIO.setup(SPICS, GPIO.OUT)

# 10k trim pot connected to adc #0
potentiometer_adc = 0;

last_read = 0       # this keeps track of the last potentiometer value
tolerance = 5       # to keep from being jittery we'll only change
                    # volume when the pot has moved more than 5 'counts'

while True:
        # we'll assume that the pot didn't move
        trim_pot_changed = False

        # read the analog pin
        trim_pot = readadc(potentiometer_adc, SPICLK, SPIMOSI, SPIMISO, SPICS)
        # how much has it changed since the last read?
        pot_adjust = abs(trim_pot - last_read)

        if DEBUG:
                print "trim_pot:", trim_pot
                print "pot_adjust:", pot_adjust
                print "last_read", last_read

        if ( pot_adjust > tolerance ):
               trim_pot_changed = True

        if DEBUG:
                print "trim_pot_changed", trim_pot_changed

        if ( trim_pot_changed ):
                set_volume = trim_pot / 10.24           # convert 10bit adc0 (0-1024) trim pot read into 0-100 volume level
                set_volume = round(set_volume)          # round out decimal value
                set_volume = int(set_volume)            # cast volume as integer

                print 'Volume = {volume}%' .format(volume = set_volume)
                set_vol_cmd = 'sudo amixer cset numid=1 -- {volume}% > /dev/null' .format(volume = set_volume)
                os.system(set_vol_cmd)  # set volume

                if DEBUG:
                        print "set_volume", set_volume
                        print "tri_pot_changed", set_volume

                # save the potentiometer reading for the next loop
                last_read = trim_pot

        # hang out and do nothing for a half second
        time.sleep(0.5)

Answer 1

The code below may help you. Please try it and tell me result.

import spidev
import time
import RPi.GPIO as GPIO

spi_ce0 = spidev.Spidev()

spi_ce0.open(0,0)

# channel1: adc1,   channel2: adc2
adc1 = spi_ce0.xfer2([1,(8+0)<<4,0])
adc2 = spi_ce0.xfer2([1,(8+1)<<4,0])

Answer 2

This is a very wide question because the problem can be a result of many things, hardware or software.

To narrow it down I would suggest to measure if there actually is any data transfer on the I/Os. If you don't have an oscilloscope it will probably be OK to use a LED (and a suitable resistor of course) to see if the chip-select pin (CS) on the MCP3002 goes low. Is there any activity on data in? Is there any answer on data out?

Data out on the MCP3002 should connect to MISO (Master In Slave Out) on the Raspberry Pi so you don't connect output to output and vice versa.

You also need to enable the SPI on the Raspberry Pi, you can do this by running "sudo raspi-config" and stepping down to "Advanced options" -> "SPI".

Have you connected the MCP3002's VDD to 3.3v from the Raspberry Pi? Be carefull not to connect it to the 5v, then you will destroy the Raspberry Pi's inputs.

Seems like you switching the clock manually without any delay. I guess the python GPIO library is quite slow so that should probably be fine. The minimum clock high time is 140nS for the MCP3002 and i would guess the python library takes you to the microsecond scale at least.