A Loop Back Example

Because of the way that data is transfered on the SPI bus it is very easy to test that everything is working without out having to add any components. All you have to do is connect MOSI to MISO so that anything send it also received in a loop back mode. 

First connect pin 19 to pin 20 using a jumper wire and start a new NetBeans project. 

The program is very simple. First we initalize the library and the SPI bus:

if (!bcm2835_init()) {
 return 1;
}
if (!bcm2835_spi_begin()) {
 return 1;
}

If either call fails the chances are you aren't running as root - see chapter 2.

As this is a loop back test we really don't need to configure the bus but for completness:

bcm2835_spi_setBitOrder(BCM2835_SPI_BIT_ORDER_MSBFIRST);
bcm2835_spi_setDataMode(BCM2835_SPI_MODE0);
bcm2835_spi_setClockDivider(BCM2835_SPI_CLOCK_DIVIDER_65536);
bcm2835_spi_chipSelect(BCM2835_SPI_CS0);
bcm2835_spi_setChipSelectPolarity(BCM2835_SPI_CS0, LOW);

Next we can send some data and recieve it right back:

uint8_t read_data = bcm2835_spi_transfer(0xAA);

The hex value AA is useful in testing because it generates the bit sequence 10101010 which is easy to see on a logic analyser 

We can check that the recieved data matches the sent data in a variety of ways:

  if( read_data== 0xAA) printf("data received correctly");

Finally we close the bus and the library:

bcm2835_spi_end();
bcm2835_close();
return (EXIT_SUCCESS);

Putting all of this together gives us the complete program:

#include <stdio.h>
#include <stdlib.h>
#include <bcm2835.h>

int main(int argc, char** argv) {

 if (!bcm2835_init()) {
  return 1;
 }
 if (!bcm2835_spi_begin()) {
  return 1;
 }

 bcm2835_spi_setBitOrder(BCM2835_SPI_BIT_ORDER_MSBFIRST);
 bcm2835_spi_setDataMode(BCM2835_SPI_MODE0);
 bcm2835_spi_setClockDivider(BCM2835_SPI_CLOCK_DIVIDER_65536);
 bcm2835_spi_chipSelect(BCM2835_SPI_CS0);
 bcm2835_spi_setChipSelectPolarity(BCM2835_SPI_CS0, LOW);

 uint8_t read_data = bcm2835_spi_transfer(0xAA);

 if( read_data== 0xAA) printf("data received correctly");

 bcm2835_spi_end();
 bcm2835_close();
 return (EXIT_SUCCESS);
}

 

If you run the program and don't get the "data received correctly" message then the most likely reason is that you have connected the wrongt two pins together or not connected them at all. 

If you connect a logic analyser to the four pins involved - 19,21, 23 and 24 you will see the data transfer: 

 

 

If you look carefully you will see the CS0 line go low before the master places the first data bit on the MOSI and hence on the MISO lines. The documentation states that the CS line is held for at least three core clock cycles before transfer starts and held for at least one clock cycle when the transfer is complete. Notice that the clock rises in the middle of each data bit making this a mode 0 transer. You can also see that the clock is measured to be 3.8KHz as priiomised. 

Problems

The SPI bus is often a real headache because of the lack of a definitive standard but in most cases you can make it work. The first problem is in discovering the characteristics of the slave device you want to work with. In general this is solved by a careful reading of the data sheet or perhaps some trial and error - see the next chapter for an example. 

If you are working with a single slave then generally things work once you have the SPI bus configuration set correctly. Where things are more difficult is if you have multiple devices on the same bus. The Pi can only directly suport two devices but this is enough to make the task more difficult. Typically you will find SPI devices that don't switch off properly when they are not being addressed. In principle all SPI devices shouldpresent high impedance outputs (i.e. tristate buffers) when not being addressed but some don't. If you encounter a problem you need to check that the selected slave is able to control the MISO line properly. 

Another problem, which is particularly bad for the Pi, is noise. If you are using a USB or other power supply to the Pi that isn't able to supply sufficient instantaneous current draw then you will see noise on any or all of the data lines - the CS0/1 lines seem to be particularly sensitive. The solution is to get a better power supply.

If you really need more than two SPI devices then you might be tempted to look at the possiblity of enabling the other SPI buses. This sounds good in theory but in practice it seems to be difficult  The documentation advises not to use the SPI bus used for EEPROM I/O in HAT boards in the Pi 2 and later. 

A better solution is to multiplex the CS0/1 lines to create additional chip selects. For example you can use standard GPIO lines as chip selects and connect more than two SPI slaves. 

Summary

• The SPI bus is often problematic because there is no SPI standard
   
• Unlike other serial buses it makes use of unidirectional connections. 
   
• The data lines are MOSI master output slave input and MISO master input slave output. 
   
• In addition there is a clock line - output from master and an unspecified number of select lines - two in the case of the Pi.
   
• Data is transferred from the master to the slave and from the slave to the master on each clock pulse in arranged as a circular buffer.
   
• The bcm2835 library provides all the functions you need to set up the SPI bus and transfer data one byte or multiple bytes at a time. 
   
• You can test the SPI bus using a simple loopback connection.
   
• Working with a single slave is usually fairly easy, working with mutliple slaves can be more of a problem.

 

 

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