HP 15C buttons are double entering numbers

[mwirth]

I bought a Limited Edition 15C a few years ago and have never been able to use it confidently because it has a tendency to double enter numbers and I have to constantly review each entry before hitting enter. I really enjoyed the 11C and 12C which never had the issue. This one was made in China. Any ideas?

[BobVA]

This post discusses a keyboard repair that worked for me, and another approach is described here.

[BillBee]

For an 11c I have used suggestions found on this site, DeOxit Red and a strand of wire (to scrape in the tiny access hole located under the rear cover). So far so good.

If it gets to the point where you are going nuts - look at the Swiss Micros DM15 - absolutely a better alternative then trying to find a minty 15c used.

-Bill

[jklsadf]

Unfortunately the key debounce firmware for the arm-based voyager calculators isn't the best. There was an "sdk" with source code released for the original arm-based hp 12c+ (the older one with the at91sam7l before the chip got discontinued), and the hardware in the hp 15c limited edition is largely the same (although with an actual packaged IC instead of chip-on-board construction in the cheaper and probably better-selling and thus higher-quantity 12c+). https://www.hpmuseum.org/forum/thread-18...16911.html

The debounce code is fairly simplistic: after detecting a new key down, the calculator sets a timer during which new detections of the same key getting newly pressed are disabled. Unfortunately, the keys can occasionally bounce for longer than the timer during normal operation, and thus register as multiple presses. This is easily observable on the both the arm-based hp 12c+ and the hp 15c limited edition. A better implementation "integrates" to detect true key downs and ups by either incrementing or decrementing a saturating counter depending on whether the key is detected as pressed or not. https://summivox.wordpress.com/2016/06/0...ebouncing/

This is more complicated, and takes more code/memory though. The last time I looked at the swiss micros key debouncing algorithm for the DM 15L (the source code for an "sdk" for those calculators is available -- or at least used to be available), it was also too simplistic.

The DM 42 from swiss micros has a new firmware now as of July 2019 (unfortunately the source code for the low-level routines such as key scanning and debouncing is not available, although they did end up releasing an "sdk"/os for the DM 42 after a lot of negative feedback from not releasing much code, but still using free42, which is GPL licensed), and the new firmware greatly improves the key debouncing routines to prevent duplicated keys due to key bouncing. Unfortunately the keyboard for the DM 42 has had a lot of other issues with the physical hardware completely unrelated to key debouncing.