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I can't seem to understand why there is a diode at the Rx pin on this Adafruit Ultimate GPS board. If we are sending data to the GPS, why put a diode that prevents current flow into this pin?

Adafruit Ultimate GPS Schematic

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    \$\begingroup\$ Are you referring to D1? Did you read the note in the schematic telling you what it's for? \$\endgroup\$ – brhans Jan 13 at 0:43
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The pull-up resistor to 3.3V and the diode means that even if you feed the circuit with 5V logic (most Arduino boards use 5V logic) the GPS chipset will see a maximum of 3.3v. The MT3339 device may be damaged or operate incorrectly if any of its pins go above its 3.3V supply rail.

If the signal RX_5V goes to a logic low the diode will conduct and pull the MT3339 RX pin to ~0.7V that it will treat as a logic low.

If RX_5V goes to 5V resistor R3 will pull the MT3339 RX up to 3.3V and the diode will be reverse biased.

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    \$\begingroup\$ and that sir is the answer!!! \$\endgroup\$ – Edwin Fairchild Jan 13 at 5:43
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The purpose of the diode is documented in a line of text near it on the schematic. The diode and resistor protect the RX input.

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  • \$\begingroup\$ I should have elaborated. Yes, I see the note on the schematic and see that if a voltage higher than 3.3V is at RX_5V then current will be blocked by D1. But I do not understand how this allows any data to be transferred to the Rx pin if D1 is blocking all current flow from RX_5V. Could it be that if RX_5V is grounded/low/off, then RX will read 0 and if RX_5V is high (~3.3V or greater), RX will read 1 while being protected from any signal >3.3V? EDIT: Just saw @Kevin White's follow-up. Thanks! \$\endgroup\$ – YNGVV Jan 13 at 1:00

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