FTDI201x and Atmega32u4

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#1
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Hello, a quick question:

 

I got a 32u4 with 5V VCC and the FTDI201x connected via TWI with 1k pull-up resistors. 1k is from the FTDI's datasheet. According to 32u4's datasheet, it needs (5V-0.4V)/3mA=~1.6k resistors.

201x's datasheet's shows that I need to connect the pull-up's to VCCIO, which is 3.3V and coming from the FTDI chip.

But if 32u4's IO is 5V, how can this work? Shouldn't I pull the SDA/SCL to 0.7*VCC=3.5V(Input High-voltage, from the twi electrical characteristics, page 387) minimum for it to count as "high" for the 32u4?

Followed is the example circuit for the USB to I2C converter from the FTDI's datasheet (owned and made by FTDI, not me). 

 


For example, UMFT201XE (Page 14) module connects the pull-ups to 3.3V aswell.

Anyone knows how should I act? I guess I need to add minimum of 1.6k resistors to SDA/SCL, but what about the logic levels?
If I have connected the pull-ups to 5V, would it even work?

 

Cheers,

T

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I would've thought that you would connect vccio to vcc and you'd get your 5V levels.
Scratch that thought - the datasheet specifically says that is verboten.

Solution is to use bidirectional level translators. A popular solution is to use mosfets in a common gate configuration. Adafruit use this on a number of their boards.

Last Edited: Mon. Jan 4, 2016 - 11:40 AM
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Thanks for the reply!

 

Can you direct me to the page number where it says VCCIO to VCC is forbidden? Nevermind, found it.

Last Edited: Mon. Jan 4, 2016 - 12:32 PM