Using ACLK and the 32kHz Crystal

The other day I decided to build a timer to control some lights in my apartment. As I hope to expand this mini project into a full-fledged home automation system built around the CC430, a simple light timer from Walmart would not do! Obviously, the LaunchPad is the perfect tool for the job.

For example, let's say that I wanted to keep my lights on for 5 hours at night while I am on vacation. The first thing you will need is a way to tell time; this requires some sort of real-time clock. Since the high frequency clock sources on the MSP430 are not accurate enough to keep reliable time over a large period of time (anything greater than 1 minute in my opinion), a lower frequency clock will be needed. Though many of the newer MSP430s have built in 32kHz clocks, such as the the MSP430F5510, the value line series does not!

The 32.768kHz Crystal

The first step for building this project is to install the 32.768kHz crystal onto the LaunchPad. From this point onward I will just be calling this 32kHz for simplicity. You might be wondering why a real time clock is based on 32.768kHz; 32768 is exactly 2^15. This number can be divided down using binary values to give you a frequency of 1Hz, or a period of 1 second. This is why 32kHz is the standard frequency used in real-time clocks.

The above image shows the crystal soldered onto my LaunchPad. There are many methods you can use to solder this on, one of which is nicely documented on Justin's Tech blog. I ended up soldering the base of the crystal first, ensuring that the clock was positioned correctly before I soldered the small leads. Use any method that works for you.

The LED Hello World

The first thing you should do once you have this soldered onto your board, is test it. Making sure that everything works before you start a complicated project is very important. Let's make an LED turn on every two seconds, for one second. Instead of changing the timer output pins directly as we did in an earlier post, lets blink the LED manually so that we can easily expand the functionality of this program in a future post. There are a few things you should notice in the code below.


In this code we divide the clock by 64 (lines 26 and 37) which causes the timer to increment 512 times a second (512Hz). Once the clock counts up to 512, one second has gone by, and an interrupt is thrown. Now that we are entering an interrupt routine instead of automatically toggling the output, we can use this interrupt routine in the future to expand the functionality of this code.

The last important line of code is line 27, where a capacitance value is set. This value matches the capacitors that come installed on your LaunchPad. In the next section, I will elaborate slightly on what this capacitance is.


Edit: This statement is actually incorrect. A big thank you goes out to Jens-Michael Gross for pointing it out to me.

"The capacitance setting is an internal switch that enables some silicon capacitors on the MSP die. The selection has to match the required load capacitance of the used watch crystal. You can set it to minimum (plain parasitic pin capacitance) and apply external capacitors of the proper value, if you want. However, the available options are sufficient for the most watch crystals, so external capacitors are unnecessary, even counterproductive. And external capacitors have a large tolerance that affects the crystal frequency. The LaunchPad I just got has no capacitors installed (the C21 and C22 pads are empty, as it should be if the XCAP options are used."

Custom Design

One thing I wanted to mention before this post comes to a close, is how you can take this design off the LaunchPad and make it your own. Many projects work out so well that you just want to create a PCB or make it permanent in some other way.

Working with crystals can be tricky for beginners, as there is one thing you must look out for. All crystals require a load capacitance to remain stable, Wikipedia (Pierce Oscillator) and Texas Instruments both have some quality information on the topic. The value of these two capacitors depend on which crystal you use. Even two crystals with the same frequency which are made by the same manufacture might require different load capacitor values. Please check the crystal's datasheet for this information. For example, the LaunchPad uses 12pF capacitors to load the crystal we just installed, yet many crystals require 22pF.


Edit: This statement is actually incorrect. Another big thank you goes out to Jens-Michael for pointing it out to me. Thanks for reading!

"12pF is the typical load for most crystals I've ever seen. But due to the electric connection, a 12pF load means [there will be 2] 24pF capacitance on each of the crystals sides. Reason is that (seen from the crystal), the two capacitors are in series to each other and parallel to the crystal. […] Subtract the ~2pF pin capacitance of the MSPs pins and you get [two separate] 22pF [for] external capacitors, resulting in 12pF load. The XCAP settings already include pin capacitance and the /2 factor."

A bit more information and great advice from Jens-Michael!

"Experiments have shown that the G devices (in opposition to the AFE2x and some other x2 family devices without HFXT1 input) will accept a high-frequency TTL clock signal (e.g. from a self-oscillating quartz oscillator) on the XTIN pin, when in bypass mode. The Datasheet limits external clock to 50kHz, but there were no problems with 16MHz."

Conclusion

As you can see by the following screenshot from my oscilloscope, this timer is pretty darn accurate.

Hopefully you enjoyed this post and found it informative. I am going to try to keep things a bit more bite-sized from now on. Let me know what you think.

Post links to your projects which use the ACLK or the 32kHz crystal in the comment section below!