The SPI bus can be something of a problem because it doesn't have a well defined standard that every device conforms to. Even so if you only want to work with one specific device it is usually easy to find a configuration that works - as long as you understand what the possibilities are. 

 

 

SPI Bus Basics

The SPI bus is very strange but commonly encountered as it is used to connect all sorts of devices from LCD displays, through real time clocks and AtoD converters.

It is strange because there is no standard for it and different companies have implemented it in different ways as a result you have to work harder to implement it in any particular case. However it does usually work which is a surprise for a bus with no standard or clear specification.

The reason it can be made to work is that you can specify a range of different operating modes, frequencies and polarities. This makes the bus slightly more complicated to use but generally it is a matter of looking up how the device you are trying to work with implements the SPI bus and then getting the Pi to work in the same way. 

The bus is odd in another way - it does not use bidirectional serial connections. There is a data line for the data to go from the master to the slave and a separate data line from the slave back to the master. That is instead of a single data line that changes its transfer direction there is one for data out and one for data in. 

It is also worth knowing that the drive on the SPI bus is push-pull and not open collector/drain. This provides higher speed and more noise protection as the bus is driven in both directions. 

There is a bidirectional mode where a single wire is used for the data - the Pi doesn't support this.

You can see the sort of configuration that the Pi expects. There is a single master and at most two slaves. The signal lines are

• MOSI Master Output Slave Input i.e. data to the slave
• MISO Master Input Slave Output i.e. data to the master
• SCLK Serial Clock which is always generated by the master

There can also be any number of SS - Slave Select - or CE Chip Select - lines which are usually set low to select which slave is being addressed. Notice that unlike other buses I2C for example there are no SPI commands or addresses - only bytes of data. However slave devices do interpret some of the data as commands to do something or send some particular data. 

The Pi has only a single SPI bus exposed on the GPIO connector and only two SS lines. This means that in principle you can only connect two SPI devices to the Pi although this is a restriction that is easy to overcome. 

 

The pins that are used for the Pi's SPI bus are 

MOSI   GPIO 10 19  Out
MISO   GPIO 9  21  In
SCLK   GPIO 11 23  Out
CE0    GPIO 8  24  Out
CE1    GPIO 7  26  In

 

The data transfer on the SPI bus is also slightly odd. What happens is that the master pulls one of the chip selects low which activates a slave. Then the master toggles the clock SCLK and both the master and the slave send a single bit on their respective data lines. After eight clock pulses a byte has been transferred from the master to the slave and from the slave to the master. You can think of this as being implemented as a circular buffer - although it doesn't have to be. 

 

This full duplex data transfer is often hidden by the software and the protocol used. For example there is a read function that reads data from the slave and sends zeros or data that is ignored by the slave. Similarly there is a write function that sends valid data but ignores whatever the slave sends. The transfer is typically in groups of eight bits and usually most significant bit first but this isn't always the case. In general as long as the master supply clock pulses data is transferred. 

Notice this circular buffer arrangement allows for slaves to be daisy chained with the output of one going to the input of the next. This makes the entire chain one big circular shift register. This can make it possible to have multiple devices with only a single chip select but it also means any commands sent to the slaves are received by each one in turn. For example you could send a convert command to each AtoD converter in turn and receive back results from each one. (See:https://www.maximintegrated.com/en/app-notes/index.mvp/id/3947)

The final odd thing about the SPI bus is that there are four modes which define the relationship between the data timing and the clock pulse. The clock can be either active high or low - clock polarity CPOL and data can be sampled on the rising or falling edge of the clock - clock phase CPHA. All combinations of these two possibilities gives the four modes:

 

SPI Mode	Clock Polarity CPOL	Clock Edge CPHA
0	0	0	Clock active high data output on falling edge and sampled on rising
1	0	1	Clock active high data output on rising edge and sampled on falling
2	1	0	Clock active low data output on falling edge and sampled on rising
3	1	1	Clock active low data output on rising edge and sampled on falling

 

The way that the modes are named is common but not universal. 

There is often a problem trying to work out what mode a slave device uses. The clock polarity is usually easy and the Clock phase can sometimes be worked out from the data transfer timing diagrams and:

• First clock transition in the middle of a data bit means CPHA=0
• First clock transition at the start of a data bit means CPHA=1

So to configure the SPI bus to work with a particular slave device you have to 

1. select the clock frequency - anything from 125MHz to 3.8KHz on the Pi.
2. set the CS polarity - active high or low
3. set the clock mode Mode0 thru Mode3

Now we have to find out how to do this using the bcm2835 library.