wiring-up: explain receiver supply at high telemetry power - #560
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Closes ExpressLRS#493. Nothing on the site connects a receiver reset to its telemetry power setting. Searching the docs for brownout, voltage drop or power draw returns nothing, and the wiring page says only to connect VCC to any 5v pad. Meanwhile the AirPort page tells the user to set receiver telemetry power to maximum, with no mention of the supply. The failure is misleading. The rail dips, the receiver resets, and the link drops and recovers, which looks like an antenna or packet rate problem. The section gives the two signs that separate a supply fault from a link fault: the problem follows the Tlm Power setting rather than distance, and the receiver restarts rather than only dropping packets. The MatchTX note is the part worth having. With MatchTX the receiver follows the transmitter power, and Dynamic Power raises transmitter power as the link degrades, so peak current draw lands exactly when the link is already weak. No current figures or regulator ratings are given. Those vary by receiver and are not derivable from the firmware, so the section stays qualitative rather than inventing numbers.
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Closes #493.
Nothing on the site connects a receiver reset to its telemetry power setting. Grepping the docs tree for
brownout,voltage droporpower drawreturns zero hits, and the wiring page says only "The VCC or 5 pad should be connected to any 5v (or 4v5) pad".Meanwhile the AirPort page actively tells the user to "set the RX telemetry power output to maximum", and notes that "Receivers with higher telemetry power output (i.e. 100mW telemetry power) will provide better results", with no mention of what that does to the supply.
Why this is worth a section
The failure mode points away from its cause. The rail dips under a transmit burst, the receiver resets, and the link drops and recovers. That presents as an antenna, packet rate or range problem, so the obvious next steps are all the wrong ones.
The section gives two signs that separate a supply fault from a link fault:
Tlm Powersetting rather than distance, so loweringTlm Poweris a direct test.The MatchTX note
This is the part I think is most useful, and it is verifiable. With
Tlm Powerset toMatchTX,DynamicPower_UpdateRx()sets receiver power fromlinkStats.uplink_TX_Power, so the receiver follows the transmitter. Dynamic Power raises transmitter power as the link degrades. Peak receiver current therefore arrives exactly when the link is already weak, which is when a marginal supply will give out.What is deliberately absent
No current figures, no regulator ratings, no "needs X mA at Y mW" table.
Those numbers vary per receiver and per amplifier and cannot be derived from the firmware or from anything else in the repositories. Issue #493 has an empty body, so there is no source for them there either. Writing plausible looking figures into the manual would be worse than leaving them out, so the section stays qualitative and actionable instead. If someone measures real figures for common receivers, they would slot straight in.