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Thread: Possible new shields

  1. #26
    As I said earlier, I tend to like what I called a hacker's shield that has islands with pins needed for a particular purpose (I2S1, I2S2, Serial1, Serial2, i2C0, i2C1, SPI0, SPI1, etc.). And it would be nice if there was an option for using standard cables (such as the 5x2 IDC cable, or the RJ45 cable with 8 wires).

    Now in your shield there isn't necessarily room for these island, but it would be useful to avoid the usual cross wiring that you see with breadboards. Obviously there are some issues, such as you can't use SPI1 and Serial1 at the same time, since they use the same pins. And with SPI pins, you often want to chose other pins for the CS, D/C, and reset pins, particularly if you have multiple SPI devices on the bus (such as flash memory, a SD card reader, and a display).

    Speaking of the CS pins, you may want to have pull-up resistors for the common pins, just like you have for the i2c pins.

    And a lot of times now, I find myself wishing I had a separate 5v power bus.

    But again that likely won't fit this into your current setup.

    Yeah, it's a pretty tight sqeeze on a 2 layer board, but it helps keep the cost down. If I add too much more, some traces are going to have 4-5 vias.... not ideal for high-speed signals. There isn't much room for any more connectors, but there are a few things I think I could fit:

    - A 5V level translator for WS2812 strings
    - 5-pin Serial port header
    - Some general purpose LEDs
    - Piezo buzzer
    - Spring terminals for the sides

    I was wondering about carving a through path for a wire under the spacer PCB to get that missing pin connected as long as the trace connects all the other pads?
    That should work. You could also find some 1mm washers and use those as spacers. I'm pretty sure the ones that come with the kit are about 1mm thick, so you could try those.

  2. #27
    Join Date
    Nov 2012
    Santa Fe, New Mexico, USA
    I have been a long time fan of the Dontronics SimmStick system. You mount your microprocessor and run all the pins out on a parallel bus to a series of SIMM sockets. Then you build extension boards with fingers and notches that mate to the SIMM sockets. Each extension board pulls only the power busses and signals that it needs and does with them what it wants. The founder of Dontronics retired at 75 last year, but parts of his website live on buried inside his daughters' scuba tourism influencer website. Ah, in fact, here's his catalog of SimmStick backplanes and plugins. Seems that passive backplanes with processor mounted on a plugin were common.

    Reading I see that credit for the original SIMM backplane idea belongs to Antti Lukats from Estonia.

    These days, the raspi compute module is built to fit a DDR2 SODIMM socket, 67.6mm x 31.1mm, and google sees multiple host boards advertised for it.

    Hmm, a 62.3mm x 18.0mm board, like a Teensy 3.6 might fit into that form factor, too. Maybe build a passive backplane with SODIMM sockets, design SODIMM carrier cards for all the Teensy 3+ that route shareable signals to shared bus lines, unique signals to unique bus lines. The same set of carrier cards might adapt stackable shields built for different Teensy models.

    I didn't realise that the Pi compute module routes all the processor gpio to the SODIMM, not just the gpio that the Pi normally exposes. Interesting.

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