0
\$\begingroup\$

I'm trying to drive a MCU input with a RS-485 driver chip, specifically the SP3485 on the SparkFun RS-485 transceiver breakout.

When I connect the MCU pin directly to the 3.3V power rail, the pin reads 3.3V and an ammeter shows 6.8 uA of current going into the pin.

I assert the RS-485 signal and measure the voltage at the SP3485 receiver output (RO) pin with the pin floating (disconnected from the MCU). I see 3.3V.

When I connect the RO pin to the MCU the MCU pin is only driven to 1V and the ammeter shows 3.7 uA of current going into the pin.

What is going on? Surely the RO output pin isn't current-limited to 3.7 uA?

\$\endgroup\$
3
  • \$\begingroup\$ When I connect that pin to the MCU the MCU pin is only driven to 1V and the ammeter shows 3.7 uA of current going into the pin. What pin do you mean, RO? How's the MCU pin connected to this RO pin configured? \$\endgroup\$ – vtolentino Apr 3 '20 at 7:39
  • \$\begingroup\$ Yes, when I connect RO to the MCU input. The connection is RO --> ammeter --> MCU input pin. The connection is direct with no pulldown resistor. I edited for clarity. \$\endgroup\$ – bcattle Apr 3 '20 at 7:43
  • \$\begingroup\$ What MCU and pins are you connecting to the SP3485? Is there any firmware running in the MCU, which could've configured the pins? \$\endgroup\$ – vtolentino Apr 3 '20 at 8:38
2
\$\begingroup\$

What have you done with the pin on the Spark Fun breakout called "RTS"?

If this signal is high at ~3.3V the RO pin of the SP3485 will be tri-stated and highly likely lead to the situation you are seeing on your MCU input at 1V.

enter image description here

Picture Source

\$\endgroup\$
1
  • \$\begingroup\$ Thanks Michael! Yeah the RTS pin is tied to ground. \$\endgroup\$ – bcattle Apr 3 '20 at 22:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.