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• Updating the Motherboard plan

  Joel Newman • 02/23/2020 at 23:06 • 0 comments

  While the first revision of the motherboard worked well there were a few things that needed to be fixed after working with the prototype. 

  
  Deficiencies noted:

  1. Inconsistent pin layout in RJ45 connectors
  2. many unused pins that could be used for other things
  3. board mounting holes are in odd location
  4. RJ45 connectors should be shielded in case shielded cables want to be used
  5. No way to tell if sensors plugged in or not
  6. GPIO extender for LED's was wrong footprint and to close to the jacks
  7. No good grounding point for a oscilloscope for testing
  8. No way to power the device without USB power
     1. Would be nice to power this from 12V and have on board converters
  9. LED lights should also be controlled from motherboard
  10. ADC had a bunch of noise on the input
      1. No can to prevent unwanted EMI
      2. No analog filtering to prevent aliasing
  11. Restricted use of pins on the microcontroller
  12. only external comms was though USB
  13. Soldering was substandard

  Objectives for next revision of the motherboard:

  1. Change the layout on the RJ45 pins to mach the PoE standard 
     1. Addresses deficiency 1
  2. Change the RJ45 sockets to be shielded to prevent EMI leakage
     1. Addresses deficiency 10
  3. On the sensor side of the circuit tie a pin from the power to a set pin (standard across all connectors) with a resistor to let the system know which device is plugged in. 
     1. It may be a good idea to include a resistor on each device so that the circuit can autodetect what kind of switch or sensor is plugged into a socket
        1. probably overkill for this project as it would make the programming much more difficult and not all jacks can support all types of signals.
     2. Need to include a GPIO extender to test all of the IO to see if its plugged in or not.
     3. Addresses deficiencies 2 and 5
  4. Analog Filter for input to ADC
     1. requires more thorough investigation on input topologies for high noise applications. Most likely going to use an example circuit from here (https://www.analog.com/media/en/technical-documentation/application-notes/an43f.pdf) to design a better bridge circuit.
  5. External power module so that usb or external 12V power can be used
     1. Addresses deficiency 8
  6. Use a standard microcontroller not a development board.
     
     1. Most likely a SAMD variant as the built in USB support makes the programming easier
     2. Addresses deficiencies 2,5,9, and 11. Possible 12 as well.
  7. Complete redesign of PCB
     1. Addresses all deficiencies

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