Wednesday, June 24, 2015

Crazy Clock accuracy problems

I've discovered a design flaw in the Crazy Clock. The result of this flaw is that the clock fairly consistently gains time at a rate of ~120 ppm. That's around ten seconds fast per day. The clock is supposed to stay within 10 ppm - more than an order of magnitude better than that.

Fortunately, the error is consistent in the samples I've taken, and can be corrected in software.

If you've got an affected clock, there are two ways to fix it:

1. If you're very confident in your surface mount soldering skills, you can add a 12 pF capacitor between each lead of the crystal and ground. I do not recommend attempting this, as there's very little room on the board, and almost no margin for error. If you ruin your board attempting this, I won't replace it.

2. The new SW_TRIM facility can be used to trim the clock frequency to compensate for the error. To do this yourself, fetch the firmware from github, compile it with -DSW_TRIM and reflash the controller. After doing that, you'll also want to set the trim factor in EEPROM. It's located at address 4 and is a two byte, two's compliment, little endian integer representing the amount of adjustment in tenths-of-a-ppm. Positive values slow the clock. The standard correction factor for this error is 0x490, or 116.8 ppm slower. The avrdude argument to achieve this is "-U eeprom:w:0xff,0xff,0xff,0xff,0x90,0x04:m". If you do this, you'll wipe out the PRNG seed at locations 0-4, so if your clock has random behavior, you should replace each of the '0xff' in that command with four random numbers from 0-255. There's no harm in not doing this, really, but your random number generator will wind up starting from a "standard" state, resulting in repeated behavior the first time you power the clock up. Don't forget that the system clock speed is 32.768 kHz, so your programming SPI clock must be no faster than 4 kHz. Achieve this by adding either -i 125 or -B 250, depending on your programmer.

If you have an affected clock, you may ship it back to me and I will apply the fix and return the clock to you for free (including shipping to get the clock back to you). If you bought a complete movement, return just the movement - keep the hands and face and stuff. If you bought a controller only, then please return just the controller. In both cases, you'll get a replacement, not necessarily the same one you send. Please be sure to indicate which firmware style you wish. To proceed, contact me with the contact form and I'll send you the address to ship your movement or controller.

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