Saturday, August 16, 2014

Crazy Clock user guide

Crazy Clock store on Tindie

This page is the permanent home for the Crazy Clock user's guide. It will be updated whenever changes are made. The hardware history:

  • v0.1 - Prototype
  • v0.2 - Changed crystal to HC-49 through-hole footprint and reduced size
  • v0.3 - Slight size reduction
  • v0.4 - Slight size reduction - downgraded over-spec'd parts.
  • v0.5 - Trade out ceramic for tantalum caps in boost section. Dramatic size reduction.
  • v0.6 - Trade out flyback diodes for a single diode array - use same array (wasting one side) for boost section - reducing BOM.
  • v0.7 - Change to a SMD 32.768 kHz crystal.
  • v0.8.x - v0.9.x - All versions the same, just reorganized for smaller sizes.
  • v1.0 & Q80 v1.0 - Same as before, but Q80 variant shaped to be drop-in replacement for Quartex movements.
  • Q80 v1.1 - Move some components for relief, add solder jumper to disconnect coil for programming (if needed).
  • v1.1 & Q80 v1.2.1 - Change crystal to 1206 footprint and add loading capacitors.
The crazy clock comes as either a bare controller board which you can use to retrofit an existing clock movement or as a complete movement with the crazy clock controller installed in place of the original (boring) controller.

To retrofit an existing clock movement, carefully disassemble the movement and remove the stepper coil and controller board, which should be a single assembly that can be easily removed. The board should be on the bottom and there should be two bare contact patches which touch two contacts that come from the battery. Two other points on the board should be soldered (directly or indirectly) to the two ends of the coil wire. For best results, do not disturb the soldered joints of those wires. Instead, use a Dremmel or an Xacto knife and cut the traces leading away from the two coil wire solder patches and the two battery contact patches. Solder 30 gauge wires to each. For best results, use a dab of hot glue to secure the wires to the board as a strain relief. Reinstall the board, routing the wires carefully to avoid interfering with any moving parts of the movement. Connect the ends of the wires to the battery and clock terminals on the crazy clock controller board. Tuck the controller board in an out-of-the-way spot inside the movement, again insuring that neither it nor any of the wires will interfere with any of the movement's moving parts. Alternatively, you can route the wires outside of the movement enclosure and use foam tape to secure the controller externally (doing it this way is less stealthy, but makes reprogramming the controller much easier). There's an instructional video that shows the major points of the process.

If you're buying a clock movement to modify with the Crazy Clock, the recommended model is the Quartex Q-80. It turns out, at least at the present time, these are the movements being used by Zazzle for their clocks. The Crazy Clock fits best as shown in this picture:



The crazy clock will run for around 12 months or so on a single AA battery, depending on the particular clock movement into which it's installed. Simply replace the battery when the clock stops. Other than the changes made to how the clock ticks, actual operation of it is exactly the same as the original movement. You set the time by rotating the set knob and replace and install the AA battery in the same manner as before.

Depending on which firmware you selected, your clock will behave differently from a normal clock, but should still keep average time accurate to within 30 seconds per month (±10 ppm), so long as the clock is kept near room temperature (around 25 °C, or 76 °F). When exposed to extreme temperature swings, you can expect the accuracy to suffer.

If you change your mind, you can re-flash the firmware. You may need to disconnect (one lead of) the stepper coil from the clock terminal before you do so, however, as the stepper pins are also used for programming. The stepper coil is a low impedance load, and that might interfere with the programmer's ability to properly drive the pins. To do this with the Q80 board, you can cut the trace between the two pads of the solder jumper near the bottom right side of the board. Once programming is complete, use a blob of solder to short the two pads together again. The firmware files are available at the Crazy Clock repository on Github. You will need a pogo pin programming adapter and an AVR programmer. You'll also need the AVR gcc and binutils toolchain. Because the controller's system clock runs at approximately 32 kHz, the SPI clock for the programmer must be at least 6 times slower. This means adding either the -i or the -B option (depending on your programmer) to AVRDUDE with a value of 250. If your programmer does not support slowing the SPI clock or if it supports doing so in some other manner, you'll need to make whatever arrangements are required.

If you want to build your own Crazy Clock controller from scratch, you'll need to fuse your ATTiny before programming it. The correct fuse settings are low:0xE6, high:0xD7 and extended:0xFF. If you bought a completed crazy clock board, its fuses will have been correctly set, and changing them is not recommended. An ATTiny45 has sufficient flash memory for any of the firmware loads, and an ATTiny25 has enough for all of them except the "Crazy clock." You can build a crazy clock equivalent on a breadboard or some other equivalent prototyping system. If you do, you will either need to use a SOT-23-5 breakout board for the NCP1402, a ready-made boost converter breakout, or just dispense with the boost converter and use some other power supply. Two AA batteries in series would work fine, as would a CR2032 (but it would only last around a month).

Firmware Operating Notes

The Crazy, Early and Warpy firmware, by their nature, drift measurably from the correct time, and the controller cannot know where the actual hands of the clock are pointing at any given time or when they are moved by the set knob. For this reason, if you wish to set the clock, you should momentarily disconnect the battery to reset the controller. This will zero any drift. For the Warpy firmware, the clock will begin the fast-running 12 hour period immediately.

The first four bytes of EEPROM on the controller are the seed for the pseudo-random number generator. Bytes 5 and 6 are active if you enable the SW_TRIM option of the firmware. The values are a little-endian, two's compliment trim value in tenths of a part-per-million. Positive values slow the clock down and negative values speed it up. To obtain the calibration value, you can either observe the clock's behavior for an extended period (although with some novelty firmware this can be impossible), or you can load the special calibration firmware image. This image will output a 16.384 kHz (nominal) square wave on one of the coil pins. You can obtain the calibration value by measuring the actual frequency with a frequency counter and determining the offset from the nominal value.

Testing & Troubleshooting

Testing the controller is quite simple. Attach a two-lead, bi-color LED, such as this one, to the "clock" terminal. In fact, the leads on a 5mm LED are spaced perfectly to fit right in. For testing, don't solder the leads, just stick them in and bend them apart a little to make a temporary connection. Similarly, connect a single AA battery, or some other power source (maximum 3 volts) to the BATT connector. Once powered, you should see the LED blink red and green with whatever pattern is appropriate for the firmware loaded.

If you don't see the LED blink, use a voltmeter and check for 3.3 volts between pins 4 and 8 of the ATtiny, or on the top and bottom of the output filter caps (the two that are next to each other to the right of the battery connector). If you see power, then the problem is likely with the microcontroller (to the right of the crystal). If not, then the problem is with the switching supply (to the left of the crystal).

For problems with the power supply, check for any components that are warm or hot to the touch, or for excessive current draw from the battery. In operation, the crazy clock should draw - on average - less than 100 µA. If it's drawing more than half a milliamp, something is likely shorting out. Also, check for any components that appear discolored or misshapen - a sure sign that the magic black smoke has escaped.

For problems with the microcontroller, check for pulses on pins 5 and 6. If you see them there, but not on the LED, then check the two series resistors and diodes for opens or shorts. If you don't see pulses coming from the microcontroller, check to make sure pin 1 (RESET) is high and use a high impedance oscilloscope probe to check for 32.768 kHz oscillations on pins 2 and 3. If you see them, then if you can, try to re-flash the firmware. If it won't accept new firmware but all of the other tests pass, then the controller itself is likely bad. When attempting to program the controller, don't forget that the SPI clock must be no faster than 4 kHz.

For most clock movements, the pulses of their original controllers are around 30-35 ms long, with an amplitude of ±1.5 volts. The default configuration of the crazy clock firmware uses 35 ms for its pulse length. If your movement is significantly different, this may not work properly. If you have an oscilloscope, you can power the original movement and observe its pulses. If your movement has a second hand that moves smoothly rather than jumps from one second to the next, then it's possible that the controller is "ticking" at 8 Hz (which simulates smooth movement). The default crazy clock firmware is incompatible with these movements, but the hardware could still be used (with different firmware) to alter how these clocks function.

Theory of operation

The battery is connected to a boost converter that will increase the voltage to a regulated 3.0 or 3.3 volts from whatever voltage the battery is giving out all the way down to around half a volt or so. The boosted voltage is fed into an ATTiny45 with a 32.768 kHz crystal. Timer 0 is set up for a prescale of 64, for a counting rate of 512 Hz. The firmware will use a special fractional counting method to divide that by 51.2, resulting in timer interrupts at 10 Hz. The SW_TRIM option further "bumps" the timer one count one direction or the other periodically depending on the trim factor. The interrupts will be used to wake the CPU from sleep mode, which will minimize power consumption. The clock marks time by sleeping and then being woken up every 10th of a second.

The clock movement is based on a lavet style stepper motor. To make the second hand tick once, you must apply pulses across the coil, but each must be the opposite polarity of the last. This can be accomplished simply by pulsing one of two digital pins high (keeping the other grounded) and connecting both to the coil. To protect the controller from coil collapse voltages, a reverse biased diode to ground from each coil wire is present. Series resistors on each wire also reduce the voltage since the original working voltage of the system was the battery's 1.5 volts instead of 3.3. Most lavet stepper coils are a little more than 200 ohms, so 200 ohms added series resistance drops the voltage roughly in half.

Although the Crazy Clock is a digital device, it falls under the FCC part 15.103(h) exemption because it's battery powered and both the boost converter and microcontroller run slower than 1.7 MHz.

Schematic





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