樹莓派上的Python延遲

Question

我試圖模擬校準研究儀器的複合動作電位。目標是以250 Hz輸出某個10μV信號。低電壓將在稍後處理，對我來說主要問題是頻率。下面的圖片顯示了我正在嘗試製作的系統的概況。

通過從活的動物的數據採集，以及處理在MATLAB中的數據，我已經由低噪聲信號，並在12位格式789倍的值。然後我使用Git克隆了以csv格式將其存儲到Raspberry Pi的存儲庫。以下是我在RPi上編寫的Python腳本。您可以跳到腳本中的def main來查看功能。

#!/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() 

該腳本幾乎按預期工作。將MCP4921 DAC的輸出引腳連接到示波器可以很好地再現信號，並正確輸出後續延遲。

不幸的是，數據點比我需要的要多得多。我能夠將信號塞入的最短時間約爲79毫秒。這是因爲在休眠功能中除以789000，我知道這對Python和Pi來說太多了，因爲讀取csv文件需要時間。但是，如果我嘗試手動創建數組，並將這些值取出而不是讀取csv文件，我可以實現6 kHz以上的頻率而不會丟失。

我的問題是

我怎樣才能得到這個信號出現在250 Hz的頻率，並從用戶的輸入可靠地降低呢？我想過在腳本中手動將789值寫入數組，然後將SPI速度更改爲適合250 Hz的值。這可以消除慢速csv閱讀器功能，但是不能減少用戶輸入的頻率。無論如何，消除對csv.read的需求將會有很大的幫助。謝謝！

Answer 1

今天早些時候發現，所以我想我會在這裏發表一個答案，以防將來有人遇到類似的問題。

由於多種原因，數據點之間的內部延遲問題無法用sleep（）解決。我落得這樣做以下

• 移動所有數學和函數調用超過了臨界循環
• 請在花費無延遲傳輸值時間的線性迴歸分析
• 將CSV文件中的數據點數量增加到MATLAB中的「很多」（9600）
• 計算滿足用戶所需信號長度所需的點數
• 從現在較大的CSV文件中取平均分離的條目以適應儘可能接近點數。
• 在臨界循環

新的代碼計算這些值，然後計算出SPI明確字節

保存兩個字節清單，並將其輸出直接，具有位輸入的檢查，是低於

#!/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() 

結果正是我想要的。以下是信號長度爲5 ms的示波器示例; 200赫茲。謝謝你的幫助，夥計們！