Article Index

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. 




Now On Sale!

You can now buy a print or ebook edition of Raspberry Pi IoT in C from Amazon.


For Errata and Listings Visit: IO Press



This our ebook on using the Raspberry Pi to implement IoT devices using the C programming language. The full contents can be seen below. Notice this is a first draft and a work in progress. 

Chapter List

  1. Introducing Pi (paper book only)

  2. Getting Started With NetBeans In this chapter we look at why C is a good language to work in when you are creating programs for the IoT and how to get started using NetBeans. Of course this is where Hello C World makes an appearance.

  3. First Steps With The GPIO
    The bcm2835C library is the easiest way to get in touch with the Pi's GPIO lines. In this chapter we take a look at the basic operations involved in using the GPIO lines with an emphasis on output. How fast can you change a GPIO line, how do you generate pulses of a given duration and how can you change multiple lines in sync with each other? 

  4. GPIO The SYSFS Way
    There is a Linux-based approach to working with GPIO lines and serial buses that is worth knowing about because it provides an alternative to using the bcm2835 library. Sometimes you need this because you are working in a language for which direct access to memory isn't available. It is also the only way to make interrupts available in a C program.

  5. Input and Interrupts
    There is no doubt that input is more difficult than output. When you need to drive a line high or low you are in command of when it happens but input is in the hands of the outside world. If your program isn't ready to read the input or if it reads it at the wrong time then things just don't work. What is worse is that you have no idea what your program was doing relative to the event you are trying to capture - welcome to the world of input.

  6. Memory Mapped I/O
    The bcm2835 library uses direct memory access to the GPIO and other peripherals. In this chapter we look at how this works. You don't need to know this but if you need to modify the library or access features that the library doesn't expose this is the way to go. 

  7. Near Realtime Linux
    You can write real time programs using standard Linux as long as you know how to control scheduling. In fact it turns out to be relatively easy and it enables the Raspberry Pi to do things you might not think it capable of. There are also some surprising differences between the one and quad core Pis that make you think again about real time Linux programming.

  8. PWM
    One way around the problem of getting a fast response from a microcontroller is to move the problem away from the processor. In the case of the Pi's processor there are some builtin devices that can use GPIO lines to implement protocols without the CPU being involved. In this chapter we take a close look at pulse width modulation PWM including, sound, driving LEDs and servos.

  9. I2C Temperature Measurement
    The I2C bus is one of the most useful ways of connecting moderately sophisticated sensors and peripherals to the any processor. The only problem is that it can seem like a nightmare confusion of hardware, low level interaction and high level software. There are few general introductions to the subject because at first sight every I2C device is different, but here we present one.

  10. A Custom Protocol - The DHT11/22
    In this chapter we make use of all of the ideas introduced in earlier chapters to create a raw interface with the low cost DHT11/22 temperature and humidity sensor. It is an exercise in implementing a custom protocol directly in C. 

  11. One Wire Bus Basics
    The Raspberry Pi is fast enough to be used to directly interface to 1-Wire bus without the need for drivers. The advantages of programming our own 1-wire bus protocol is that it doesn't depend on the uncertainties of a Linux driver.

  12. iButtons
    If you haven't discovered iButtons then you are going to find of lots of uses for them. At its simples an iButton is an electronic key providing a unique coce stored in its ROM which can be used to unlock or simply record the presence of a particular button. What is good news is that they are easy to interface to a Pi. 

  13. The DS18B20
    Using the software developed in previous chapters we show how to connect and use the very popular DS18B20 temperature sensor without the need for external drivers. 

  14. The Multidrop 1-wire bus
    Some times it it just easier from the point of view of hardware to connect a set of 1-wire devices to the same GPIO line but this makes the software more complex. Find out how to discover what devices are present on a multi-drop bus and how to select the one you want to work with.

  15. SPI Bus
    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. 

  16. SPI MCP3008/4 AtoD  (paper book only)

  17. Serial (paper book only)

  18. Getting On The Web - After All It Is The IoT (paper book only)

  19. WiFi (paper book only)



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:

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
Clock Edge
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.