If you have read my blog at all you may be beginning to think I have a fetish for the MCP9808. Well, maybe a little, but for a good reason. If we look it’s data sheet we see the MCP9808 is a surprisingly complex device. Some of the features the data sheet lists:
- ±0.25 (typical) from -40°C to +125°C
- ±0.5°C (maximum) from -20°C to 100°C
- ±0.5°C (maximum) from -20°C to 100°C
- User-Selectable Measurement Resolution:
- +0.5°C, +0.25°C, +0.125°C, +0.0625°C
- User-Programmable Temperature Limits:
- Temperature Window Limit
- Critical Temperature Limit
- User-Programmable Temperature Alert Output
- Operating Voltage Range: 2.7 V to 5.5V
- Operating Current: 200 µA (typical)
- Shutdown Current: 0.1 µA (typical)
- I2C bus compatible
These features make the MCP9808 ideal for a variety of temperature monitoring applications when being precise would be an advantage.
The devices I have been using I got from Adafruit. They have a driver library for the Arduino that is useful, but I want to use the device with Linux based IoT platforms such as the Intel Edison or the Beaglebone Black. Both of these platforms support the Intel UPM library , so it made sense to add a driver for the MCP9808 to that. I implemented the driver and added a pull request to have it incorporated into the Intel UPM library. The MCP9808 was accepted in to version 4.0 of the library and should be available after upgrading the version of MRAA on your board. If not — I have a blog post that lists various ways to update MRAA/UPM on your boards. (UPM is very easy to compile).
I provided a C++ program to the UPM repo that exercises all of the implemented functions. You can copy the code from that example into an Intel iotdk-ide project (Eclipse) and run it from there. But I didn’t include a very good example for Nodejs. That brings us to the purpose of this post — I want to go over how to use the MCP9808 in Nodejs with XDK on the Edison (or compatible) MRAA board.
In order to give a proper demo I created a Nodejs/Socket.io app that illustrates all the implemented features of the MCP9808 UPM driver. It is setup as an XDK project so you can download it and open it in the XDK ide to run it on your Edison. Before you run it open the server.js file and change the line
var temp = new mcp.MCP9808(1)
to whatever i2c bus you are using. I generally use a handmade boardthat extends the Edison Mini breakout board so I use bus 1. It will be bus 6 on the Arduino or DFRobot breakout, 0 on the Galileo.
When you download and start the app on the board browse to it’s url :8085 (like edison.local:8085) and you should see something that looks like this:
This is a single page app that uses socket.io to communicate with the Edison. The Edison serves the app via Express and the UI is basic HTML with the Bootstrap framework. The four sections of the app (from right to left) are:
The radial gauge will reflect the current temperature at the device. The MCP9808 reports temperatures as Celsius (C), but the driver provides a conversion to Fahrenheit (F). The F values are calculated, so when we switch between C and F there are sometimes rounding differences. All the other temperature values in the UI will convert to C or F except for resolution. Resolution is always reported as C. I have the temperature limited from freezing to boiling range, but the MCP9808’s range is little wider than that — see the specsheet.
These reflect the state of the three temperature limit registers detailed in section 5.1.2 of the spec sheet. These registers are used to allow the setting of temperature limits the MCP9808 can monitor so that we don’t have to constantly check for out of limit temps in code. The limit registers will cause flags in the ambient temp register (section 5.1.3) to set if a threshold has passed. The driver exposes these bits as flags that can be read (see lines 158 to 167 of the server code) via a simple function call. The thing to remember with these flags is that they reflect the state of the bits as of the most recent getTemp() call, so you need to read the temp before reading the flags.
I use the state of the Temp limit bits to set the color of the subheadings in the Temp Limit section. Since at power on the temps are set at 0 C, so the bits for TCrit and TUpper are set. Using the sliders to set a value greater than the current temperature will allow the TCrit and TUpper values to go green:
The temp limit registers are also used in the Alert functionality.
Described in section 5.2.3 of the spec sheet, the MCP9808 has the ability to control an alert pin when certain conditions arise. On power up alerts are disabled. Default sets the device to comparator mode (section 22.214.171.124). The alert will assert (in this case pull low — you need to use a pull up on the pin) whenever one of the monitor temps are crossed. When the temp comes back within limits the alert will de-assert. In interrupt mode the just the TUpper and TLower are monitored. It the temp goes over a threshold the alert is asserted and will not de-assert until the temp goes back with range and the interrupt is cleared by code. These calls are illustrated in the server code.
The Other section contains controls for resolution and hysteresis and display for device info, id and rev.
Resolution will always be reported in Celsius. Startup default is 0.0625 and equates to 4 temp measurements per second. Temperature resolution is detailed in section 5.2.4 of the spec sheet.
Hysteresis will be reported in either C of F depending on the driver settings. Hysteresis is detailed in section 5.2.2 and Figure 5-10. If the alerts are not clearing as you expect, check the hysteresis settings.
The about menu item is a list of helpful links concerning the MCP9808.
What’s not Illustrated:
I didn’t use the sleep and wake function in this application. It is illustrated in the test file in the UPM library.
What’s not implemented:
There were a couple of things that I did not implement. First: The temp monitoring registers can be locked by setting bits in the config register. It takes a power cycle to unlock them so I didn’t implement this. The alert can be set to assert active high. I didn’t add this functionality either. If either of these capabilities are needed you can just make an MRAA call directly to the device to do so. If you need help with either of these just ask me in the comments.