Skip to main content

Deglitching a Sparkfun 7-segment Serial Display

The display in my Ambient Bus Arrival Monitor is a Sparkfun 7-segment Serial Display connected to the TTL serial port.  I had noticed that occasionally the display would reset itself to 0000 (or sometimes 0, 00 or 000).  It was even possible to make it do this by touching the body of the bus.  It didn't happen often so I was able to ignore it but then it began to happen more and more.

After a very long and tedious investigation I discovered why.  I started out by blaming my code, my soldering, the cable I was using, the quality of the connectors, ...  Only having eliminated everything that I'd touched did I realize it must be something else.

The display has two input methods: serial (which I am using) and SPI (which I am not).  The SPI interface has a clock signal (which in the case of the display is acting as an input) called SCK.  The manual for the display says "The display is configured using SPI mode 0 (CPOL = 0, CPHA = 0), so the clock line should idle low".

If you take a quick look at the schematic for the display you'll discover that the SCK pin on the microcontroller is just connected to a solder pad for connection.


The upshot is that if you (the user) don't connect that pin to something Sparkfun aren't doing it for you.  The display glitch I was seeing was that this floating clock input would sometimes go high and the firmware for the device would then read a byte (all zeroes of course) and write to the display.

There were two possible fixes: hack the firmware so that it ignores SPI completely, or force the SCK pin low all the time.  I opted for the latter (since it was a quick fix) and connected a 10k resistor between that pin and ground.  Glitch gone.



Pity that Sparkfun didn't include a pull-down resistor on SCK (and possibly on RX as well).

Comments

DJ Scrim said…
Excellent! I thought there would be a snowstorms chance in hell of finding any info on this bug, but then I found your post.

I am employing roughly 10 of these into products monthly and have also been battling this issue.

Thanks a mill, I owe you a beer. Preferably an IPA.

Cheers!
DJ Scrim said…
Excellent! I thought there would be a snowstorms chance in hell of finding any info on this bug, but then I found your post.

I am employing roughly 10 of these into products monthly and have also been battling this issue.

Thanks a mill, I owe you a beer. Preferably an IPA.

Cheers!
I'll accept the IPA as payment.

Popular posts from this blog

Your last name contains invalid characters

My last name is "Graham-Cumming". But here's a typical form response when I enter it: Does the web site have any idea how rude it is to claim that my last name contains invalid characters? Clearly not. What they actually meant is: our web site will not accept that hyphen in your last name. But do they say that? No, of course not. They decide to shove in my face the claim that there's something wrong with my name. There's nothing wrong with my name, just as there's nothing wrong with someone whose first name is Jean-Marie, or someone whose last name is O'Reilly. What is wrong is that way this is being handled. If the system can't cope with non-letters and spaces it needs to say that. How about the following error message: Our system is unable to process last names that contain non-letters, please replace them with spaces. Don't blame me for having a last name that your system doesn't like, whose fault is that? Saying "Your

All the symmetrical watch faces (and code to generate them)

If you ever look at pictures of clocks and watches in advertising they are set to roughly 10:10 which is meant to be the most attractive (smiling!) position for the hands . They are actually set to 10:09.14 if the hands are truly symmetrical. CC BY 2.0 image by Shinji I wanted to know what all the possible symmetrical watch faces are and so I wrote some code using Processing. Here's the output (there's one watch face missing, 00:00 or 12:00, because it's very boring): The key to writing this is to figure out the relationship between the hour and minute hands when the watch face is symmetrical. In an hour the minute hand moves through 360° and the hour hand moves through 30° (12 hours are shown on the watch face and 360/12 = 30). The core loop inside the program is this:   for (int h = 0; h <= 12; h++) {     float m = (360-30*float(h))*2/13;     int s = round(60*(m-floor(m)));     int col = h%6;     int row = floor(h/6);     draw_clock((r+f)*(2*col+1), (r+f)*(row*2+1),

The Elevator Button Problem

User interface design is hard. It's hard because people perceive apparently simple things very differently. For example, take a look at this interface to an elevator: From flickr Now imagine the following situation. You are on the third floor of this building and you wish to go to the tenth. The elevator is on the fifth floor and there's an indicator that tells you where it is. Which button do you press? Most people probably say: "press up" since they want to go up. Not long ago I watched someone do the opposite and questioned them about their behavior. They said: "well the elevator is on the fifth floor and I am on the third, so I want it to come down to me". Much can be learnt about the design of user interfaces by considering this, apparently, simple interface. If you think about the elevator button problem you'll find that something so simple has hidden depths. How do people learn about elevator calling? What's the right amount of