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Topics - jefflongo

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Discussion - Hardware / FT232R CBUS pull-up
« on: January 15, 2020, 03:35:57 AM »
Hello all,

I am using FT232R's CBUS3 pin in I/O mode (with VCCIO=3.3v) to control the reset pin of an STM32 microcontroller (which has an integrated 45kohm pull-up to vdd = 3.3v). When I am not actively driving the reset pin, I set CBUS3 to input mode. The FT232R datasheet specifies that input pins have an internal 200kohm pull-up to VCCIO. However, I notice that when I set CBUS3 to input mode, the reset node voltage drops from 3.3v to 2.6v. After disconnecting CBUS3 from the circuit, the reset node voltage is back to 3.3v, and CBUS3 is in fact not pulled-up when floating in input mode. Furthermore, if CBUS3 was even just tri-stated, I would expect the microcontroller's embedded reset pull-up to pull the voltage to 3.3v.

I have ensured the EEPROM is programmed to set CBUS3 to I/O mode and that the pull-down on USB suspend is not checked. My application works despite the reset node voltage dropping, but this is still less than desirable. Does anybody have any idea why CBUS3 is causing the voltage to drop?

Best,
Jeff

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