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VQB 71 GPS Clock

A tiny version of the GPS Wall Clock

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A small clock using vintage VQB 71 segment displays from East Germany. Time comes from space courtesy of a NEO-6M GPS module. This project is the little brother of GPS Wall Clock.

Details and copious photos in the project logs:

tiny_gps_vqb_71_rev1.zip

KiCAD project for Rev 1.0

x-zip-compressed - 63.32 kB - 03/29/2020 at 05:21

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tiny_gps_rev1_schematic.png

Rev 1.0 schematic image

Portable Network Graphics (PNG) - 952.01 kB - 03/29/2020 at 05:06

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  • Ticking accurately with the NEO-6M

    Stephen Holdaway08/07/2021 at 01:11 1 comment

    Like any normal person, I want a desk clock that has sub-millisecond accuracy.

    Parsing and immediately displaying time from NMEA messages as they arrive from the GPS module, like GPS Wall Clock originally did, gives a useable-enough value for display, but it's not particularly accurate for a few reasons:

    • The GPS module outputs messages at 1Hz, but each of these serial transmissions only begins once the GPS has finished its internal fix calculations. Fix calculations take a variable amount of time depending on the number of satellites being used in the calculation, which can be from 40 to 100ms for 1-12 satellites.
    • At the default 9600 baud, a NMEA RMC sentence containing the current time will take 42 - 82ms to receive, depending on the number of fields filled.
    • Converting the received time into segment data and sending to the MAX7219/MAX7221 over SPI takes some processing time.

    To avoid these delays and uncertainties, the GPS module offers a TIMEPULSE pin that can be used to generate interrupts on our controller at the top of each second with considerable accuracy.

    GPS PPS vs serial message timing
    Even with a good GPS signal, there's a significant delay between the tick of each second and the serial data containing that time.

    How accurate are we talking?

    The u-blox documentation claims it's possible to get timing accuracy in the order of nanoseconds with ideal conditions, while accuracy in the order of microseconds is quite achievable:

    As a rule of thumb the position should be known with an accuracy of better than 1m for a timing accuracy in the order of nanoseconds. If an accuracy is required only in the order of microseconds, a position accuracy of roughly 300m is sufficient.

    Let's not get too carried away though - we're making a display for humans after all, where perception seems to take in the order of tens of milliseconds at best. In practice it will take someone much longer to look at and interpret the display.

    Consider that in your day-to-day life, knowing the time to ±1 minute would likely be inconsequential, and ±1 second would be practically imperceptible. I would hazard a guess that the standard deviation of maintained time-keeping devices you encounter is at least 30 seconds, though I can't find anyone who has sampled this, and networked clocks probably tighten this guess up a bit.

    I'm happy drawing a line at ±1ms, mainly because digging any deeper yields more and more uncertainty:

    • What is the precision of the timepulse signal? (Something like ±30ns if the jitter and quantization in the module's 48MHz clock isn't compensated for)
    • How quickly does segment data sent to the MAX7221 actually change the output signal? (The datasheet lists a maximum output propagation delay of 25ns)
    • How fast do the LED segments actually light up? (It depends, but probably <1μS)
    • How long does it take for light to travel from the display to your eyes? (about 3.3ns, if you're 1 metre away).
    • ...ad nauseam ...

    Given that I'm using these modules indoors and don't have equipment to verify absolute accuracy anyway, all of this is essentially academic, but it's fun to consider.

    Using timepulse (1PPS)

    The TIMEPULSE pin on the GPS module generates a rising edge at the top of each second, with a configurable offset in nanoseconds to account for electrical and processing delays. Messages sent by the module always correspond to the previous timepulse, though the documentation only mentions this a few times briefly:

    NEO-6M top of second (PPS) vs serial data out (from the Receiver Description and Protocol Specification PDF)
    Order of the timepulse and serial data output (u-blox 6 Receiver Description, p27)

    To display the time correctly, we'll need to calculate the next time to display when a NMEA "Recommended Minimum" (RMC) message arrives. At the following rising TIMEPULSE edge, this calculated time needs to be displayed. The MAX72xx will immediately update its display when any digit data is written, so the most I can prepare in my case is internally buffering segment data ready for blasting out over SPI.

    Due to the reversed wiring...

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  • Design and bring-up of a VQB 71 GPS clock

    Stephen Holdaway03/29/2020 at 05:15 2 comments

    A year ago I picked up some new-old-stock "VQB 71" 7-segment displays — not for any particular project, but simply because they looked fantastic. These gorgeous, vintage common-anode digits have distinctive segments with six tiny LEDs each, internally connected with bond-wires and encapsulated in transparent red epoxy/resin:

    Macro photo of a VQB 71 digit, powered off
    Microscope photo of an unpowered VQB 71, lit to reveal its internal features. Hundreds of tiny bubbles can be seen in the red epoxy under this lighting, which aren't visible to the naked eye. The digit height within the display is 7mm.

    This batch was manufactured in 1989* by WF Berlin (Werk für Fernsehelektronik), a division of the East German state-owned company VEB Kombinat Mikroelektronik Erfurt. The packaging is titled "lichtemitteranzeige" which is literally "light emitting display": an odd name by modern standards according to a German colleague of mine, who said this would simply be labelled "sieben segmentanzeige" today.

    Tray of 20 RFT Lichtemitteranzeige VQB 71 7-segment LED displays made by WF Berlin in June 1989
    VQB 71 displays in packaging
    * The marking "X06" indicates a manufacture date of June 1989, assuming Wikipedia is correct about the manufacturer using IEC 60062 letter and digit codes. WF Berlin appears to have phased out most of its manufacturing in 1990 (source), so a mid-1989 date makes sense for new-old-stock parts.

    A few partial scans of datasheets for the VQB 71 are available online, with the one below from tu-chemnitz.de being the most complete copy I've found. A datasheet isn't strictly necessary here, but it's useful to have dimensions provided for the footprint, and I wanted to know the maximum current per segment specified by the manufacturer. After all, These parts haven't been made for nearly 30 years, so it would be a shame to destroy one accidentally.

    VQB 71 Datasheet
    VQB 71 datasheet scan. The contents of this image is transcribed below.

    It's easy enough to figure out the specs without knowing any German, but Google Translate makes quick work of the dry/technical language. Here's the English translation of this datasheet:

    VQB 71
    Not for new designs
    Light emitting display made of red-beam GaAsP diodes in a segment design to display the digits 0...9 and a decimal point (DP). The wavelength of their max emission is 630...690 nm. The half width is 40 nm.

    Limits  
    Forward direct current / segment or DP at θa -25...25 °CIFmax 15 mA
    DC blocking voltage at θa -25...+70 °CURmax 4 V
    Operating temperature θa -25...+70 °C
    Storage temperature for storage up to 30 days θstg -50...+70 °C
    Characteristic values at θa = 25°C  
    Luminous intensity / segment at IF = 10 mAIVtyp 150 μcd
    ↳ Decimal pointIVtyp 100 μcd
    Forward DC voltage / segment at IF = 10 mAUFmax 3.6 V
    ↳ Decimal pointUFmax 1.8 V

    Note that unlike modern LED datasheets, this doesn't specify maximum pulsed currents. Pulsed currents with modern LEDs are typically 3-5x the max constant current rating (dependent on duty cycle and pulse duration), which can give some head-room for driving them harder in multiplexed configurations.

    Driving these segments with a constant 10mA as the datasheet suggests gives a high-contrast image in vivid red. The VQB 71's are so intensely red that it's proved difficult to capture an accurate representation of one on camera. Modern LEDs like those in the GPS Wall Clock look almost orange by comparison.

    VQB 71 with one segment powered by a constant 10mA source
    Segment powered with a constant 10mA source. Note that the segment is completely over-exposed here. With no light cast on anything, it's difficult to convey the true brightness.
    VQB 71 exposed for segment at 10mA constant current
    Same as above, but exposed for the segment - a reduction of 7 whole stops.

    Driving all segments at this power increases the surface temperature of the digit by around 10°C, but who needs efficiency when you have style, right?

    I hear we're building another clock

    There are only so many applications for a 7-segment display these days, and I'm a firm believer that one person can never have enough clocks. A miniature version of...

    Read more »

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