Chapter D – Serial Connections to the RPi and Analog-to-Digital Converters http://www.esologic.com/?p=665

The MCP 3008 A/D converter The MCP3008 has 8 analog inputs that can all be converted to digital values between 0 and 1023 (there are 10 bits of data), and: uses sample and hold circuits to get the data runs on voltages between 2.75 – 5.5 V can transfer between 75 – 200 ksps transfers data with the SPI protocol (0,0 or 1,1)

SPI pins on the RPi GPIO Pin 19 Master Out / Slave In Pin 21 Master In / Slave Out Pin 23 Serial Clock Pin 24 Chip Enable 0 (or Chip Select 0) Pin 26 Chip Enable 1 (or Chip Select 1)

More on the RPi SPI pins The Serial clock controls the speed at which serial data is transferred between the RPi and the A/D. The serial clock period is an integer multiple of the CPU clock period. You can change the multiple between 2 and 65,536 (by powers of 2). This must be adjusted for the speed of the A/D. The Chip select lines are used to select an A/D. With two CS/CE lines, you could select from four serial devices. A device is selected when the Chip select is low (usually).

More on the RPi SPI pins The Master Out / Slave In is normally the line over which data will be sent from the RPi to the A/D. This line will tell the selected A/D : to start sending data to chose between single-line mode and differential input mode. from which of its 8 analog inputs to send data. The Master In / Slave out is normally the line that will be receiving data from the A/D.

Accessing the MCP3008 A/D converter VDD Supply voltage VREF Reference voltage AGND analog ground CLK clock input Din data in Dout data out CS/SHDN chip select DGND digital ground CH0 – CH7 analog inputs

A/D – RPi connections MCP 3008 RPi RPi Pin# VDD  3.3V 01 VREF  3.3V 01 AGND  GROUND 06 CLK  SCLK 23 Din  MOSI 19 Dout  MISO 21 CS/SHDN  CE0 24 DGND  GROUND 06 CH0 – CH7 goes to temperature sensor!

Communication process CE0 must be low RPi must send 5 bits of information to A/D on MOSI: A “1” to indicate that data should be sent A 0 or 1 to indicate whether differential data should be sent, or single line data, respectively (we want 1) Three bits to indicate which analog input should be digitized and read: 000 for CH0 010 for CH2 100 for CH4 001 for CH1 011 for CH3 …111 for CH7

Communication process, ctd So, to read Channel zero, we need to send “11000”. This can appear anywhere in a byte, so: 00011000 = 0x18  we will use this one 00110000 = 0x30 01100000 = 0x60 would this one be better? 11000000 = 0xC0

Communication process, ctd Thus, sample channel calls are: CH0 00011000 = 0x18 CH1 00011001 = 0x19 CH2 00011010 = 0x1A CH3 00011011 = 0x1B CH6 00011110 = 0x1E CH7 00011111 = 0x1F

Converting the digital data The A/D has a precision of 10 bits, or 210=1024 The 10 bits are divided between two bytes (dc=don’t care): data[1] data[2] We are going to right shift B3 – B0 4 bits: We are going to mask 6 bits: We will left-shift the left byte 4 spaces by multiplying by 16 and add the right byte in an integer (2 bytes long): data_int X B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 dc X B9 B8 B7 B6 B5 B4 X B3 B2 B1 B0 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0

Sample code for the MCP3008 // mcp2835 example #2 // using a temperature sensor to measure ambient temp // working with the MCP3008 A/D converter // #include <bcm2835.h> #include <stdio.h> // codes needed for the eight analog channels #define ANALOG0 0x18 #define ANALOG1 0x19 #define ANALOG2 0x1A #define ANALOG3 0x1B #define ANALOG4 0x1C #define ANALOG5 0x1D #define ANALOG6 0x1E #define ANALOG7 0x1F

The MCP3008.c program continued int main(int argc, char **argv) { int Count, data_int; float data_voltage, temperature; char channel[4]={0,0,0,0}, data[4]; // // check for proper initialization of the GPIO if (!bcm2835_init()) { printf("error initializing the GPIO"); return 1; }

The MCP3008.c program continued // // Set up spi serial communication. // Slow down transfer rate (divide clock frequency by 4096). // Use CS0/CE0 to initiate data transfer (when CS0 is set to zero). // Use SPI serial interface mode 0,0 (MODE0). bcm2835_spi_begin(); bcm2835_spi_setClockDivider(BCM2835_SPI_CLOCK_DIVIDER_4096); bcm2835_spi_chipSelect(BCM2835_SPI_CS0); bcm2835_spi_setChipSelectPolarity(BCM2835_SPI_CS0, 0); bcm2835_spi_setDataMode(BCM2835_SPI_MODE0); channel[0]=ANALOG3; // read from analog pin3

The MCP3008.c program continued // // main loop to read data // Send data on which channel to use and then receive 4 bytes of data. // The first two bits are DC (Don't care), then there are 10 bits of // data from MSB to LSB, then there are 10 bits of data LSB to MSB. // The rest of the bits from the MCP3008 are zero. while(1) { bcm2835_spi_transfernb(&channel[0], &data[0], 4); // convert to integer, voltage, temperature: the next three lines require // considerable explanation data_int=16*(data[1] & 0x3F) + ((data[2]>>4)&0x0F); data_voltage=3.3*data_int/1024.; temperature=72.+100.*(data_voltage-0.75);

The MCP3008.c program final lines // // print out results printf("\nData out= "); for (Count = 0; Count < 4; Count++) printf("%02X ",data[Count]); printf(" %i %f %f",data_int, data_voltage,temperature); } //end of loop - Return SPI pins to default inputs state bcm2835_spi_end(); return 0;