Raspberry Pi IoT In C - AtoD With The SPI Bus

Written by Harry Fairhead

Monday, 01 August 2016

Article Index
Raspberry Pi IoT In C - AtoD With The SPI Bus
Connecting And Getting Data

Page 2 of 2

Connecting MCP3008 To The Pi

The connection to the PI's SPI bus is very simple and can be seen in the diagram below.

diagram

The only additional component that is recommended is a 1uF capacitor connected between pins 15 and 16 to ground mounted as close to the chip as possible. As discussed in the previous section you might want a separate voltage reference for pin 15 rather than just using the 3.3V supply.

Basic Configuration

Now we come to the configuration of the SPI bus.

We have some rough figures for the SPI clock speed - 10kHz to a little more than 1.35MHz. So an initial clock divider of 4096 giving a frequency of 61kHz seems a reasonable starting point.

From the data sheet the CS has to be active low and the most significant bit first is the default for both the master and the slave. The only puzzle is what mode to use? This is listed in the data sheet if you look really carefully and it can be mode 0,0 with clock active high or mode 1,1 with clock active low.

For simplicity we can use mode 0,0 which is mode0 in the bcm2835 library.

We now have enough information to initialize the slave:

bcm2835_spi_setDataMode(BCM2835_SPI_MODE0); bcm2835_spi_setClockDivider( BCM2835_SPI_CLOCK_DIVIDER_4096); bcm2835_spi_chipSelect(BCM2835_SPI_CS0); bcm2835_spi_setChipSelectPolarity( BCM2835_SPI_CS0, LOW); bcm2835_spi_setBitOrder( BCM2835_SPI_BIT_ORDER_MSBFIRST);

The Protocol

Now we have the SPI initialized and ready to transfer data but what data do we transfer? The SPI bus doesn't have any standard commands or addressing structure. Each device responds to data sent in different ways and sends data back in different ways. You simply have to read the data sheet fo find out what the commands and responses are.

Reading the data sheet might be initially confusing because it says that what you have to do is send five bits to the slave - a start bit, a bit that selects its operating mode single or differential and a three bit channel number. The operating mode is 1 for single ended and 0 for differential. So to read channel 3 i.e. 011, in single ended mode you would send the slave

11011xxx

where xxx means don't care. The response from the slave is that it holds its output in a high impedance state until the sixth clock pulse it then sends a zero bit on the seventh followed by bit 9 of the data on clock eight. That is the slave sends back:

xxxxxx0b9

where x means indeterminate. The remaining 9-bits are sent back in response to the next nine clock pulses. This means you have to transfer three bytes to get all ten bits of data.

This all makes reading the data in eight bit chunks confusing.

The data sheet suggests a different way of doing the job that delivers the data more neatly packed into three bytes. What it suggests is to send a single byte

00000001

the slave transfers random data at the same time which is ignored. The final 1 is treated as the start bit. If you now transfer a second byte with most significant bit indicating single or differential mode, then a three bit channel address and the remaining bits set to zero the slave will respond with the null and the top two bits of the conversion. Now all you have to do to get the final eight bits of data is to read a third byte:

transfer1

You can do it the first way that the data sheet describes but this way you get two neat bytes containing the data with all the low order bits in their correct positions.

Using this information we can now write some instructions that read a given channel. For example to read channel zero we first send a byte set to 0x01 as the start bit and ignore the byte the slave transfers. Next we send 0x80 to select single ended and channel zero and keep the byte the slave sends back as the high order two bits. Finally we send a zero byte so that we get the low order bits from the slave i.e.

char buf[] = {0x01,0x80,0x00}; char readBuf[3]; bcm2835_spi_transfernb(buf,readBuf,3);

Notice you cannot send the three bytes one at a time using transfer because that results in the CS line being deactivated between the transfer of each byte.

To get the data out of readBuf we need to do some bit manipulation:

int data= ((int)readBuf[1] & 0x03) << 8 | (int) readBuf[2];

The first part of the expression extracts the low three bits from the first byte the slave sent and as these are the most significant bits they are shifted up eight places. The rest of the bits are then ored with them to give the full 10 bit result.

To convert to volts we use:

float volts=(float)data*3.3f/1023.0f;

assuming that VREF is 3.3V.

In a real application you would also need to convert the voltage to some other quantitiy like temperature or light level.

Some Packaged Functions

This all works but it would be good to have a function that read the AtoD on a specified channel:

int readADC(uint8_t chan){ char buf[] = {0x01,(0x08|chan)<<4,0x00}; char readBuf[3]; bcm2835_spi_transfernb(buf,readBuf,3); return ((int)readBuf[1] & 0x03) << 8 | (int) readBuf[2]; }

Notice that this only works if the SPI bus has been initialized and set up correctly. An initialization function is something like:

void SPI_init(){ bcm2835_spi_setDataMode(BCM2835_SPI_MODE0); bcm2835_spi_setClockDivider( BCM2835_SPI_CLOCK_DIVIDER_4096); bcm2835_spi_chipSelect(BCM2835_SPI_CS0); bcm2835_spi_setChipSelectPolarity( BCM2835_SPI_CS0, LOW); bcm2835_spi_setBitOrder( BCM2835_SPI_BIT_ORDER_MSBFIRST);}

How Fast

Once you have the basic facilities working the next question is always how fast does something work. In this case we need to know what sort or data rates we can achieve using this AtoD converter.

The simplest way of finding this out is to use the fastest read loop, for channel 5 say:

for(;;){ int data=readADC(0x5); }

With the clock divider we used earlier:

BCM2835_SPI_CLOCK_DIVIDER_4096

we get a measured clock rate of 61kHz the sampling rate is measured to be 2.26kHz. This is perfectly reasonable as it takes at least 24 clock pulses to read the data. Most of the time in the loop is due to the 24 clock pulses so there is little to be gained from optimization.

Increasing the clock rate to around 900kHz by setting the divider to 256 pushes the sampling rate to 36kHz which is just fast enough to digitize audio as long as you don't waste too much time in the loop in processing.

Changing the clock rate to 2Mhz, divider 128, pushes the sampling up to 70kHz which is plenty fast enough for most audio.

The fastest sample rate achieved with the samples of the device to hand was a clock rate of 4Mhz, divider 32, and a sampling rate of 216kHz - however the readings became increasingly unreliable in the low order bits. Perhaps this could be improved with the use of a buffer and careful layout but for a prototype board a sampling rate of 70kHz is the limit. Also notice the clock rate goes up you have to ensure that the voltage source is increasingly low impedance to allow the sample and hold to charge in a short time.

Summary

Making SPI work with any particular device has four steps

discover how to connect the device to the SPI put this is a matter of identifying pin outs and mostly what chip selects are supported.
find out how to configure the Pi's SPI bus to work with the device. This is mostly a matter of clock speed and mode.
identify the commands that you need to send to the device to get it to do something and what data it sends back as a response.
find or workout what the relationship between the raw reading, the voltage and the quantity the voltage represents is.

cover3

Raspberry Pi And The IoT In C

By Harry Fairhead

The new edition of this book is now available:

Buy from Amazon.

All of the articles in this list are in the process of being updated from the first edition to the second edition.

Chapter List

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.
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?
Input and Interrupts
Memory Mapped I/O
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.
PWM
I2C
I2C Temperature Measurement
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.
Getting On The Web - After All It Is The IoT Coming Soon
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.
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.
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.
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.
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.
SPI MCP3008/4 AtoD Coming Soon