75 lines
2.6 KiB
Markdown
75 lines
2.6 KiB
Markdown
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Electrical Engineering Change Notes
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===================================
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Revision 1.0
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------------
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### Description
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First revision
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### Manufactured
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Boards ordered from Aisler on January 6th 2023.
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Made directly from kicad files in r1.0 directory.
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Two of the boards were assembled and tested.
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### Issues noticed
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* Leaks from the 5V into the FPGA IOs cause two issues:
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* The 3V3 rail rises triggering a "turn-on" of the soft-off circuit
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* The FPGA goes into a weird mode where its IOs are not properly
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configured because it sees voltage on its IO before being properly
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powered.
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Theses leaks come from two path:
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* The `CHG_FPGA_n` line through the LED
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* The `SENSE` line from the voltage divider
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* The soft-off circuit wouldn't actually turn-off.
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* When the battery is not connected, the system can't be operated. The
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charging circuits charges C16 but when something starts drawing current,
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it stops and takes to much time to "start again" and C16 depletes.
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That causes the FPGA to misconfigure / misbehave.
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* Soldering the battery to raw pads isn't practical, it's useful to be able
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to disconnect it. So using a proper connector would be better.
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* The sound output doesn't really work. The speaker have too low impedance
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and so they can't be driven from the IO directly at any reasonable volume.
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That feature was abandonned.
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### Reworks
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To make them fit for purposes, some changes were applied to both assembled
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boards.
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* Replace R18 with a diode to fix the 5V leak. The `CHG_FPGA_n` can be
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internally pulled up and will be dropped to about 0.6v ( < Vil ) by
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the charge chip when charging.
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* Replace the R14/R17 divider by a N-mosfet to fix the 5V leak.
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Wired as G = `5V`, S = `GND`, D = `SENSE`.
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When 5V is present, it can charge the mosfet gate and force `SENSE` to
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ground. And then also use internal pull-up for that line. The 5V rail
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may need an active discharge resistor to make sure the gate discharges
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when USB is disconnected. Not tested.
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* Replaced C14 with a 2.2uF cap in parallel with a 100k resistor.
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This makes the `PWR_EN` line only raise to 1.6V which is enough to enable
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the regulator but makes it easier to discharge without it re-triggering.
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Note that this is still a bit dodgy and in further revisions, the whole
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circuit should be overhauled.
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* Soldered a 47uF (ZRB18AR60J476ME01L) in parallel to C16.
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Not perfect and seems flash programming can fail sometimes, but works
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most of the time for quick battery-less tests.
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* Used Molex PicoBlade 53047-0210 as connector for the battery.
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It fits in the footprint that was on board and in the space available.
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