1. Chapter D – Serial Connections to the RPi and Analog-to-Digital Converters

2. 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)

3. 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)

4. 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).

5. 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.

6. 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

7. 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!

8. 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

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

10. Communication process, ctd
Thus, sample channel calls are:
CH = 0x18
CH = 0x19
CH = 0x1A
CH = 0x1B
CH = 0x1E
CH = 0x1F

11. 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

12. 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

13. 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; }

14. 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

15. 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= *(data_voltage-0.75);

16. 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;