Files and images referenced in p3

files/measure.py

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+#!/usr/bin/python
+#-------------------------------------------------------------------------------
+# Name:        MCP3002 Measure 3V3
+# Purpose:     Measure the 3V3 Supply of the Raspberry Pi
+#-------------------------------------------------------------------------------
+
+import spidev # import the SPI driver
+from time import sleep
+
+DEBUG = False
+vref = 3.3 * 1000 # V-Ref in mV (Vref = VDD for the MCP3002)
+resolution = 2**10 # for 10 bits of resolution
+calibration = 0 # 38 # in mV, to make up for the precision of the components
+
+# MCP3002 Control bits
+#
+#   7   6   5   4   3   2   1   0
+#   X   1   S   O   M   X   X   X
+#
+# bit 6 = Start Bit
+# S = SGL or \DIFF SGL = 1 = Single Channel, 0 = \DIFF is pseudo differential
+# O = ODD or \SIGN
+# in Single Ended Mode (SGL = 1)
+#   ODD 0 = CH0 = + GND = - (read CH0)
+#       1 = CH1 = + GND = - (read CH1)
+# in Pseudo Diff Mode (SGL = 0)
+#   ODD 0 = CH0 = IN+, CH1 = IN-
+#       1 = CH0 = IN-, CH1 = IN+
+#
+# M = MSBF
+# MSBF = 1 = LSB first format
+#        0 = MSB first format
+# ------------------------------------------------------------------------------
+
+
+# SPI setup
+spi_max_speed = 1000000 # 1 MHz (1.2MHz = max for 2V7 ref/supply)
+# reason is that the ADC input cap needs time to get charged to the input level.
+CE = 0 # CE0 | CE1, selection of the SPI device
+
+spi = spidev.SpiDev()
+spi.open(0,CE) # Open up the communication to the device
+spi.max_speed_hz = spi_max_speed
+
+#
+# create a function that sets the configuration parameters and gets the results
+# from the MCP3002
+#
+def read_mcp3002(channel):
+    # see datasheet for more information
+    # 8 bit control :
+    # X, Strt, SGL|!DIFF, ODD|!SIGN, MSBF, X, X, X
+    # 0, 1,    1=SGL,     0 = CH0  , 0   , 0, 0, 0 = 96d
+    # 0, 1,    1=SGL,     1 = CH1  , 0   , 0, 0, 0 = 112d
+    if channel == 0:
+        cmd = 0b01100000
+    else:
+        cmd = 0b01110000
+
+    spi_data = spi.xfer2([cmd,0]) # send hi_byte, low_byte; receive hi_byte, low_byte
+
+    # receive data range: 000..3FF (10 bits)
+    # MSB first: (set control bit in cmd for LSB first)
+    # spidata[0] =  X,  X,  X,  X,  X,  0, B9, B8
+    # spidata[1] = B7, B6, B5, B4, B3, B2, B1, B0
+    # LSB: mask all but B9 & B8, shift to left and add to the MSB
+    adc_data = ((spi_data[0] & 3) << 8) + spi_data[1]
+    return adc_data
+
+try:
+    print("MCP3002 Single Ended CH0,CH1 Read of the 3V3 Pi Supply")
+    print("SPI max sampling speed = {}".format(spi_max_speed))
+    print("V-Ref = {0}, Resolution = {1}".format(vref, resolution))
+    print("SPI device = {0}".format(CE))
+    print("-----------------------------\n")
+
+    while True:
+        print (read_mcp3002(0), read_mcp3002(1))
+
+
+except KeyboardInterrupt: # Ctrl-C
+    spi.close()
+
+def main():
+    pass
+
+if __name__ == '__main__':
+    main()