Python延迟了Raspberry Pi

时间:2016-11-01 13:06:22

标签: python raspberry-pi delay

我正在尝试模拟校准研究仪器的复合动作潜力。目标是以250 Hz输出一定的10μV信号。低电压将在稍后处理,对我来说主要的问题是频率。下图显示了我正在尝试制作的系统的概述。

enter image description here

通过从活体动物获取数据,并在MATLAB中处理数据,我制作了一个低噪声信号,具有12位格式的789个值。然后我克隆了存储库,我使用Git将其以csv格式存储到Raspberry Pi中。下面是我在RPi上编写的Python脚本。您可以跳过脚本中的def main来查看功能。

#!/usr/bin/python

import spidev
from time import sleep
import RPi.GPIO as GPIO
import csv
import sys
import math

DEBUG = False
spi_max_speed = 20 * 1000000
V_Ref = 5000
Resolution = 2**12
CE = 0

spi = spidev.SpiDev()
spi.open(0,CE)
spi.max_speed_hz = spi_max_speed

LDAQ = 22
GPIO.setmode(GPIO.BOARD)
GPIO.setup(LDAQ, GPIO.OUT)
GPIO.output(LDAQ,GPIO.LOW)

def setOutput(val):
    lowByte = val & 0b11111111 #Make bytes using MCP4921 data sheet info
    highByte = ((val >> 8) & 0xff) | 0b0 << 7 | 0b0 << 6 | 0b1 << 5 | 0b1 << 4
    if DEBUG :
        print("Highbyte = {0:8b}".format(highByte))
        print("Lowbyte =  {0:8b}".format(lowByte))
    spi.xfer2([highByte, lowByte])

def main():
    with open('signal12bit.csv') as signal:
        signal_length = float(raw_input("Please input signal length in ms: "))
        delay = float(raw_input("Please input delay after signal in ms: "))
        amplitude = float(raw_input("Please input signal amplitude in mV: "))
        print "Starting Simulant with signal length %.1f ms, delay %.1f ms and amplitude %.1f mV." % (signal_length, delay, amplitude)
        if not DEBUG : print "Press ctrl+c to close."
        sleep (1) #Wait a sec before starting
        read = csv.reader(signal, delimiter=' ', quotechar='|')
        try:
            while(True):
                signal.seek(0)
                for row in read: #Loop csv file rows
                    if DEBUG : print ', '.join(row)
                    setOutput(int(row)/int((V_Ref/amplitude))) #Adjust amplitude, not super necessary to do in software
                    sleep (signal_length/(data_points*1000) #Divide by 1000 to make into ms, divide by length of data
                sleep (delay/1000)
        except (KeyboardInterrupt, Exception) as e:
            print(e)
            print "Closing SPI channel"
            setOutput(0)
            GPIO.cleanup()
            spi.close()

if __name__ == '__main__':
    main()

此脚本几乎按预期工作。将MCP4921 DAC的输出引脚连接到示波器,可以很好地再现信号,并正确输出后续延迟。

不幸的是,数据点的分离比我需要的要多得多。我可以将信号塞入的最短时间约为79 ms。这是因为在睡眠函数中除以789000,我知道从Python和Pi中要求太多,因为读取csv文件需要时间。但是,如果我尝试手动制作阵列,并将这些值放在外而不是读取csv文件,我可以实现超过6 kHz的频率而不会丢失。

我的问题是

如何使此信号以250 Hz的频率出现,并从用户的输入中可靠地降低?我已经考虑过手动将789值写入脚本中的数组,然后将SPI速度更改为适合250 Hz的任何值。这将消除慢速csv阅读器功能,但是你无法降低用户输入的频率。无论如何,消除对csv.read的需求会有很大帮助。谢谢!

1 个答案:

答案 0 :(得分:1)

今天早些时候弄清楚了,所以我想我会在这里发布一个答案,以防将来遇到类似的问题。

由于多种原因,睡眠()无法解决数据点之间内部延迟的问题。我最终做的是以下

  • 所有数学和函数调用移出关键循环
  • 对传输值无延迟的时间进行线性回归分析
  • 将CSV文件中的数据点数量增加到&#34;很多&#34; (9600)在MATLAB中
  • 计算满足用户所需信号长度所需的点数
  • 从现在较大的CSV文件中获取均匀分隔的条目,以尽可能接近该点数。
  • 计算这些值,然后明确计算SPI字节
  • 保存两个字节列表,并在关键循环中直接输出

新代码,带有一些输入检查,位于

之下
#!/usr/bin/python

import spidev
from time import sleep
import RPi.GPIO as GPIO
import sys
import csv
import ast

spi_max_speed = 16 * 1000000 # 16 MHz
V_Ref = 5000 # 5V in mV
Resolution = 2**12 # 12 bits for the MCP 4921
CE = 0 # CE0 or CE1, select SPI device on bus
total_data_points = 9600 #CSV file length

spi = spidev.SpiDev()
spi.open(0,CE)
spi.max_speed_hz = spi_max_speed

LDAQ=22
GPIO.setmode(GPIO.BOARD)
GPIO.setup(LDAQ, GPIO.OUT)
GPIO.output(LDAQ,GPIO.LOW)

def main():

    #User inputs and checking for digits
    signalLengthU = raw_input("Input signal length in ms, minimum 4: ")
    if signalLengthU.isdigit():
        signalLength = signalLengthU
    else:
        signalLength = 4

    delayU = raw_input("Input delay after signal in ms: ")
    if delayU.isdigit():
        delay = delayU
    else:
        delay = 0

    amplitudeU = raw_input("Input signal amplitude in mV, between 1 and 5000: ")
    if amplitudeU.isdigit():
        amplitude = amplitudeU
    else:
        amplitude = 5000

    #Calculate data points, delay, and amplitude
    data_points = int((1000*float(signalLength)-24.6418)/12.3291)
    signalDelay = float(delay)/1000
    setAmplitude = V_Ref/float(amplitude)

    #Load and save CSV file
    datain = open('signal12bit.csv')
    read = csv.reader(datain, delimiter=' ', quotechar='|')
    signal = []
    for row in read:
        signal.append(ast.literal_eval(row[0]))

    #Downsampling to achieve desired signal length
    downsampling = int(round(total_data_points/data_points))
    signalSpeed = signal[0::downsampling]
    listlen = len(signalSpeed)

    #Construction of SPI bytes, to avoid calling functions in critical loop
    lowByte = []
    highByte = []
    for i in signalSpeed:
        lowByte.append(int(i/setAmplitude) & 0b11111111)
        highByte.append(((int(i/setAmplitude) >> 8) & 0xff) | 0b0 << 7 | 0b0 << 6 | 0b1 << 5 | 0b1 << 4)

    print "Starting Simulant with signal length %s ms, delay %s ms and amplitude %s mV." % (signalLength, delay, amplitude)
    print "Press ctrl+c to stop."
    sleep (1)

    try:
        while(True): #Main loop
            for i in range(listlen):
                spi.xfer2([highByte[i],lowByte[i]]) #Critical loop, no delay!
            sleep (signalDelay)
    except (KeyboardInterrupt, Exception) as e:
        print e
        print "Closing SPI channel"
        lowByte = 0 & 0b11111111
        highByte = ((0 >> 8) & 0xff) | 0b0 << 7 | 0b0 << 6 | 0b1 << 5 | 0b1 << 4
        spi.xfer2([highByte, lowByte])
        GPIO.cleanup()
        spi.close()

if __name__ == '__main__':
    main()

结果正是我想要的。下面是示波器的示例,信号长度为5 ms; 200赫兹。谢谢你的帮助,伙计们!

Oscilloscope reading