In my previous post I started a conversation around the details of sensing a temperature with a RaspberryPi. That post discussed using the cheapest temperature sensor available: a thermistor. Using a thermistor with an RPi is cheap, but not very straight-forward: an ADC is required, we need to communicate with that ADC (over SPI in my particular case), a voltage divider circuit is required to convert a varying resistance into a varying voltage, and we need some funky math in order to account for the sensor's non-linearity with respect to resistance versus temperature over a range of temperatures. Not to mention the further adjustments we need to make in order to account for the self-heating of the thermistor device itself.
However, learning how to work with one resistive sensor opens the door to working with a whole host of other resistive sensors (light sensors, pressure sensors, potentiometers, sliders, joysticks, etc). So while reading temperature in this way is challenging, the tricks learned along the way are useful in other common electronic designs as well.
My examples use a RaspberryPi specifically because I wanted to show an end-to-end example of reading an actual temperature with an actual SBC. From evaluating various temperature devices, choosing and buying one… all the way to connecting it to the RaspberryPi and reading the temperature in user-space. However, any of a large number of SBCs could be used. Although these posts are RaspberryPi-specific, many of the steps and concepts can be applied to a wide range of Linux-capable SBCs.
Continuing along with the discussion of reading a temperature with a RaspberryPi, this post will discuss a different class of temperature sensor.
Digital Thermometer
At the polar opposite end of the spectrum from the hard-to-work-with thermistor are a class of temperature-sensing devices I'm calling "digital thermometers". I've seen that phrase used in a couple places, but the term isn't universal.
Whereas simple devices, such as the thermistor, require you to add various other components to your design and perform a complicated conversion from resistance to temperature, a digital thermometer is usually wired directly to the RPi and all that you need to do is read a temperature value directly from the device itself. There are no hoops to jump through to account for self-heating, or 3rd-order transfer functions to implement to adjust for the non-linear relationship between changing resistance versus changing temperature, no extra components to add to your design, and no circuitry you need to figure out and measure to improve the accuracy of your calculation.
The digital thermometer handles all of these things itself and simply provides a temperature (or something that is easily translated into a temperature). Conceptually it's as if all the pieces that were discussed in the previous post (except for the inter-chip communication) were encapsulated together into the sensor (including some of the software that would have needed to be written for the RPi):
NOTE: I'm not trying to imply that digital thermometers are based on thermistors; in fact according to their datasheets they are not. But I'm trying to relate back to case of the thermistor to show how much more advanced these devices are.
Examples of digital thermometers include:
- TMP102 from Sparkfun
- Si7021 from Adafruit
- TC72
- DS18B20
There are dozens (or hundreds?) of digital thermometers from which to choose. Select the accuracy, the range of temperatures you want to measure, your favourite inter-IC communication bus, and you're mostly done. Many of these devices have come about thanks to the maker movement. Taking sophisticated ICs, putting them on breadboard-friendly break-out boards, and adding any required circuitry makes successful projects easier. In addition to the hardware, most of these devices also come with (mostly Python) libraries, just in case neither hardware nor software are your strong suits when working with electronics ;-)
TMP102
The TMP102 temperature break-out board from Sparkfun has the following characteristics:
- a resolution of: 12 bits
- a range of: -25°C to +85°C
- an accuracy of: ±0.5°C
- and communicates over: I2C
If you decide to forgo the Python library you'll need to know the format of the data you'll receive from the device. Simply refer to the datasheet and you'll find:
The upper-case Ds represent the value to the left of the decimal point, the lower-case ds represent the fractions. If a whole-number calculation of the device's temperature will suffice, simply take the first byte that is given and ignore the second.
Si7021
The Si7021 is an example of a digital thermometer from Adafruit. It's specs are:
- configurable from 11-bit to 14-bit resolution
- -10°C to +85°C
- ±0.4°C
- I2C
The Si7021 provides an example of a digital thermometer that doesn't, exactly, give you a temperature, but it gives you something (a "Temp_Code") that is easy to convert to a temperature:
TC72
I can't find any examples of the TC72 on a maker-friendly break-out board but there are generic break-out boards to which it could be attached.
- 10-bit
- -40°C to +85°C
- ±2°C
- SPI
Data format:
DS18B20
The DS18B20 is a very popular temperature-sensing device, and with good reason. In addition to its great specs, it is available both as a discrete component, as well as in a water-proof temperature-appropriate housing.
- configurable 9-bit to 12-bit resolution
- -55°C to +125°C
- ±0.5°C accuracy
- 1-wire communication protocol
It's data format is as follows, and is very similar to a 16-bit signed 2's complement representation:
Designing for Multiple Sensors
In many situations the goal is to put one temperature sensor in an area (e.g. to measure the temperature of a room). For these situations, hooking up one temperature sensor to one SBC and distributing one set of these in each area will suffice. However, in other situations, the goal is to distribute a lot of sensors in a small area (e.g. monitoring the supply and return temperatures of a multi-zone HVAC system). Hooking up 1 or 2 sensors via I2C or SPI to one RaspberryPi isn't too hard, but trying to connect a dozen or more devices on these same buses can be a challenge. Another option would be to use a dozen RaspberryPis with 1 or 2 sensors each, but that's getting silly.
The problem lies with how these buses were designed, and the assumptions that were used when designing them.
SPI
The SPI bus consists of 1 clock line and 2 data lines (one for each direction):
The above diagram shows a master with one SPI bus that has 3 chip selects on it. The standard RaspberryPi 40-pin expansion header has two SPI buses. The first SPI bus (SPI0) exposes 2 chip selects, and the second SPI bus (SPI1) exposes 3 chip selects.
Pins 19 and 21 are the 2 data lines for SPI0, pin 23 is the SPI0 clock pin, and pins 24 and 26 are the 2 chip selects for SPI0.
Pins 38 and 35 are the 2 data lines for SPI1, pin 40 is the SPI1 clock pin, and pins 12, 11, and 36 are the chip selects for SPI1.
All of the pins for SPI0 are readily available on the 40-pin header. The data lines and clock pins are available for SPI1 as well. However, the 3 chip selects for SPI1 are not the primary functions of those pins, so extra setup is required to set those chip select pins for SPI mode.
If you wanted to hook up more than 2 devices to the RaspberryPi's SPI0 bus you would need more chip select lines; the same is true for the Pi's SPI1 bus as well. Setting up more chip select lines requires some hardware work and tweaks to the driver software. It's possible, but it's getting complicated due to the nature of how the SPI bus works.
Another way to connect multiple devices to an SPI bus is as follows:
This is an example of connecting multiple SPI devices in a daisy-chain topography. One chip select always selects all devices at the same time, but the data from the first device travels to the next device down the chain, and so on, until the last device connects back to the master. This is the topology used for JTAG. In theory a large number of devices can be connected to one SBC using this topology. However, with this design it's not possible for the master to query any one device at a time, it must always clock enough times to get all the data from all the devices with each request. The slaves themselves must also support a pass-through mode which enables them to save up the received bits and send them along after they've put their own data on the data line.
In other words, this topology is also complicated and not always possible.
Whether you are using the first topology (multiple chip select lines) or have daisy-chained your devices together, there is no way for your software to know how you've wired everything together. Give 10 people the same two SPI devices to connect to the RaspberryPi's SPI0 bus and half of them will connect device A to chip-select 0 and the other half of them will connect device B to chip-select 0. Neither arrangement is wrong, but these differences need to be taken into account. As a result, SPI buses are not discoverable. There is no way for Linux to know, a-priori, the devices to which it is connected or how they've been wired up. There's no unique ID that can be sent to let Linux know exactly what's connected to where. Therefore, there's no way Linux can do all the work for you; you need to write some code for your specific arrangement, and therefore are responsible for interpreting the data you receive from each device. Plus this code needs to be flexible enough to allow the user to specify the wiring so your code knows where to look for its data.
I2C
With I2C there is 1 data line and 1 clock line:
Notice that there are no chip select lines. The master specifies the slave with which it wishes to communicate by providing an address as part of the message preamble. This has the added side-effect of reducing the data throughput rate since some part of every communication is taken up specifying an address. The original I2C spec specifies 7 bits to be used for these addresses; meaning there are only 128 unique address possibilities. With 1000s and 1000s of I2C devices on the market, you can be assured the possibility for address collisions is more than probable! This address isn't based on topology or related in any way to a device's position on the bus. This address is a number that is burned into the device, and when the device sees this number in the preamble of a message, it responds. Each I2C device has to know the address to which it is to respond. For many I2C devices an address is chosen by the manufacturer and simply burned into its circuitry. This address is specified in the device's datasheet. There is no central repository or authority dispensing these addresses, so manufacturers are free to use whatever address they want. Fancier I2C devices use from one to three external pins to allow the user to modify some part of the I2C address; the rest of the address is burned into the device. When the device is placed into its circuit these pins are either grounded or tied high to change the address; at which point their address becomes static. In recent years there have been new revisions of the I2C spec that allow for 10-bit addresses, but the adoption rate has been slow.
Therefore, trying to attach multiple of the exact same device to a RaspberryPi becomes tricky. If you're lucky you might be able to change 3 of the 7 address pins, giving you the ability to add up to 8 of the same device to each I2C bus your SBC exposes… but only if you're lucky and your specific device provides these address pins. Otherwise each I2C bus on your SBC can only accommodate up to 1 instance of a specific device if that device uses a fixed address.
The standard 40-pin expansion header on the RaspberryPi includes 1 I2C bus: I2C1. Pin 3 is the I2C1 data line, and pin 5 is the I2C1 clock pin.
This situation (regarding I2C device addressing) means that the I2C bus is not discoverable. You can't connect a random bunch of I2C devices to your SBC and have Linux magically know which devices you have connected, and therefore automatically know how to interpret the data it receives from those devices. Any address could conceivably be used by any number of non-related devices, and with devices where the user can modify parts of the address itself, those devices could have any of a range of addresses. Therefore when using I2C Linux can help you with the minutiae of sending and receiving individual data packets, but knowing which devices are in use and how to interpret their data is up to you.
1-Wire
As we've already seen, the DS18B20 is one of the simplest devices to use with the RaspberryPi; wire it up, and read its temperature. No extra circuitry required, no fancy conversions. But the fact it uses the 1-wire protocol gives us a couple additional benefits as well.
As part of the 1-wire protocol, any device that uses the 1-wire protocol must have a 64-bit unique serial ID (address) burned into it from the factory. The first 8-bits of this 64-bit number identify the device class, which is centrally administered in order to avoid address collisions. A centrally-administered database of device classes means that OSes, like Linux, can uniquely identify what type of device is connected, and can therefore know how to present its data. The DS18B20's class ID is 0x28, therefore any time Linux detects a 0x28 device connected to it via 1-wire, it knows this is a DS18B20 and can dynamically create a file in sysfs that presents user-space with a simple temperature reading. 1-wire buses are discoverable.
The other advantage of the 1-wire protocol (made possible as a consequence of the unique 64-bit number burned into every device at the factory) is its ability to enumerate all such devices attached to a single bus. Due to the low-speed nature of the bus, and various other electrical properties, 100s (if not 1000s) of devices can be connected together on 1 GPIO pin; all uniquely addressable, and all able to report their unique temperature reading to the user.
Multiple Sensors Conclusion
If you want to connect only one sensor to your device, any sensor using any bus will do. Once you get between 2 and 8, you'll probably want to drop SPI and try either I2C (if your device has 3 address pins) or 1-wire. But once you getting up into the 6 range, and certainly past 8, you'll want to switch to 1-wire and use the DS18B20. Also, SPI and I2C are buses that are designed for on-device, short range communication; they are not buses that can accommodate placing sensors a significant distance away from the SBC itself.
Therefore, for designs where multiple sensors need to be spread out in an area but preferably connected to one SBC, the DS18B20 has many advantages:
- the 1-wire bus is a long-distance bus that can let you connect sensors that are several metres away from your SBC
- the DS18B20 has a wide temperature range, and high accuracy
- the DS18B20 comes in several form-factors, one of which is in a temperature-appropriate and water-proof housing
- you can connect 100s of them up reliably to a single RaspberryPi or other SBC
- due to their unique serial numbers with a centrally-managed device prefix, Linux (and other OSes) are able to know what devices are connected to them (they're discoverable) and therefore do all the conversions internally for you, presenting user-space with a simple file to read containing each device's temperature (one file per device) automatically
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