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• Interactive mode, Graphical UI or done for now??

  John Lonergan • 4 days ago • 1 comment

  At this stage the project has almost served it's purpose.

  There's one more thing I might invest in and that's a graphical UI for the interacgtive mode across the serial interface.

  It would make the exploratory side of things a lot easier.

  Toggle pin states, apply, see results etc.

  Python or Java I suppose - any suggestions?

  The interactive mode is now complete along with various options. I've included a serial monitor that uses the readline library to make usage a lot more convenient.

• Video - Interactive Mode and Usage

  John Lonergan • 4 days ago • 0 comments

  Take a look 

  https://www.youtube.com/watch?v=RRU0BgRXUZg&t=21s
  

• High-Z test possible using only 1 GPIO pin?

  John Lonergan • 02/10/2020 at 23:10 • 0 comments

  I use currently dedicate two GPIO pins per test pin in this device and this allows me to distinguish between 1/0 and Z outputs.

  I was wondering if I'd missed a trick, and whether I could have done this reliably with only one pin plus a pull down resistor.

  See simulation .. http://tinyurl.com/usm9eqd

  Firtsly, I don't think this idea works at all if I want to support TTL because I need an external pull down resistor who's value is able to pull-down a high-Z line but without interfering with reading a high logic level. That means we need a high value.

  However if we use a very high resistance for the pull-down then the input low leakage current (out) of the devices will ensure that we are not able to achieve a 0.8v low anymore.

  However if we use a low value then this will pull down the line so much that we can't achieve a valid logic high.

  This page explains pullup/down value selection and the problems associated with poor choices   https://www.electronics-tutorials.ws/logic/pull-up-resistor.html (recomended).
  

  Turns out the max pull-down for TTL is 2k !! This is assuming an input leakage current of 400uA and a max valid low threshold of 0.8v   Eg 74LS245 is 200uA 

  By contrast the MCP23017 has Input a leakage current of 1uA , 74HCT is 0.1uA, ATmega328p is 1uA which might allow for a pull down of something like 800k perhaps. 

  However,  even 800k would probably pull the logic high down to 4.4v and since this device is already pushing it a bit with voltage levels I'm not keen on taking a device that I've found reliable in testing and then making it flakey.

  So - no after all.

• Video - Principle of Operation

  John Lonergan • 02/10/2020 at 03:56 • 0 comments

  This video discusses the principle of operation of the device and provides a few CircuitJS sims of the modes of operation of the test pins.

  See https://www.youtube.com/watch?v=wdYZv8Z_g2I

• Interactive Demo of Principles of Operation

  John Lonergan • 02/08/2020 at 20:24 • 0 comments

  This tool demonstrates how the two GPIO pins are able to distinguish a tristate high-Z state (or empty socket) from a bistate high/low output.

  The demo is linked off the github page. 

  But see the Principles of Operation on github for how it works.
  

  (Built using Falsted/Circuitjs)

• PCB for free!

  John Lonergan • 02/08/2020 at 00:40 • 0 comments

  I have a box of the v1 PCBs for this project.

  If someone wants one and postage from London isn't excessive then drop me a message.

  These boards work fine as long as you seat the chip in the second row of the zif as there was a problem on row one due to me misunderstanding pins A6 and A7 of the Arduino

  Cheers John

• Interactive

  John Lonergan • 02/07/2020 at 17:14 • 0 comments

  Interactive mode now working so this will be the basis of coms for a companion program that will run on a PC

• Using PROGMEM

  John Lonergan • 02/07/2020 at 17:12 • 0 comments

  Massive reduction in dynamic memory needs (IE ram) due to moving all strings to flash. Much more stable now.

  Created a convenience library using macros  for writing to serial.print as my old functions approach no longer works with the flash strings.

• Minor PCB rework

  John Lonergan • 02/04/2020 at 12:47 • 0 comments

  I reworked the board to allow for a wide ZIF socket as there are wide "universal" Zif sockets that allow 0.3 and 0.6 chips giving more versatility.

  Also corrected a hardware error where I had mistakenly taken INTA and INTB on the MCP23017 as inputs rather than outputs. The previous hardware tied these to Vcc using a 10k resistor. The earlier revision of the board seems to work in practice despite this, however now I've left these INT outputs as unconnected.

  Also, I didn't like the way that the layout software was working so I made a minor mod. My PCB has wide power tracks and in some cases I have MCP23017 contol lines tied to Vcc or Gnd and this meant the fat trace came right up to those pins which cluttered the layout and caused clearance problems. So I included a 0ohm resistor in front of those pins to terminate the layout rule and drop back to a narrow trace. In the finished product this will be a jumper. Perhaps I could have done the same by using pad to separate the two traces? Anyway, now I have no problems with clearances.

• Why didn't I get a better ZIF socket?

  John Lonergan • 02/03/2020 at 11:00 • 0 comments

  The socket I have can hold only narrow (0.3) chips.

  There are sockets out there that will hold both narrow and wide (0.6) chips.

  This would have allowed me to test my wide pinned 7 segment LED's, or memory chips, as well as 7400 logic.

  Another thing learned I guess.

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