[英]Python delays on Raspberry Pi
我正在嘗試模擬校准研究儀器的復合動作潛力。 目標是以250 Hz輸出一定的10μV信號。 低電壓將在稍后處理,對我來說主要的問題是頻率。 下圖顯示了我正在嘗試制作的系統的概述。
通過從活體動物獲取數據,並在MATLAB中處理數據,我制作了一個低噪聲信號,具有12位格式的789個值。 然后我克隆了存儲庫,我使用Git將其以csv格式存儲到Raspberry Pi中。 下面是我在RPi上編寫的Python腳本。 您可以跳過腳本中的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()
這個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,並正確輸出后續延遲。
不幸的是,數據點的分離比我需要的要多得多。 我可以將信號塞入的最短時間約為79 ms。 這是因為在睡眠函數中除以789000,我知道從Python和Pi中要求太多,因為讀取csv文件需要時間。 但是,如果我嘗試手動制作陣列,並將這些值放在外而不是讀取csv文件,我可以實現超過6 kHz的頻率而不會丟失。
我的問題是這個
如何使此信號以250 Hz的頻率出現,並從用戶的輸入中可靠地降低? 我已經考慮過手動將789值寫入腳本中的數組,然后將SPI速度更改為適合250 Hz的任何值。 這將消除慢速csv閱讀器功能,但是你無法降低用戶輸入的頻率。 無論如何,消除對csv.read的需求會有很大幫助。 謝謝!
今天早些時候弄清楚了,所以我想我會在這里發布一個答案,以防將來有人遇到類似的問題。
由於幾個原因,睡眠()無法解決數據點之間內部延遲的問題。 我最終做的是以下內容
下面是帶有一些輸入檢查的新代碼
#!/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。 下面是示波器的示例,信號長度為5 ms; 200赫茲。 謝謝你的幫助,伙計們!
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