1
\$\begingroup\$

Reading the datasheet of the FT232R from FTDI did not answer my question so I'm asking here:

I have the following configuration:

VCC: +5V
VCCIO: +3.3V

The Part I'm unsure:

The datasheet of the FT232 states the following:

Integrated level converter on UART and CBUS
for interfacing to between +1.8V and +5V
logic.

and a maximum input voltage as:

IOmax = VCC + 0.5V

My question is:

Am I damaging the FT232 if my input high voltage is +5V on either RXD/TXD? Because I think I won't because of the stated ''feature'' but I want to be sure.

datasheet for the ft232r

EDIT If you want a reason why I'm specifically asking then: sparkfun sells these FT232 adapters which can be either 3.3V or 5V. But the only difference between the two is that either 3.3V or 5V is connected to VCCIO. Since using 3.3V on VCCIO does not affect the maximum input voltage then I would be able to use a +5V system to send and probably receive data too. This means the 3.3V version would be much more universal than the 5V version. (because I don't want to use two boards)

\$\endgroup\$
  • \$\begingroup\$ The correc question would be: Why is your VCCIO is 3.3V, when you are using the 5V levels? \$\endgroup\$ – Eugene Sh. Jan 13 '16 at 21:07
  • \$\begingroup\$ Including a link to the datasheet of a device in the question helps folks to answer quickly. If Vcc = 5V, and IOmax = VCC + 0.5V, what is the problem you expect from driving RXD/TXD to 5V? \$\endgroup\$ – gbulmer Jan 13 '16 at 21:08
  • \$\begingroup\$ @gbulmer because VCCIO is for driving the IOs \$\endgroup\$ – d3L Jan 13 '16 at 21:11
  • 1
    \$\begingroup\$ It is an unusual configuration, so it caught my attention above most other parts. So I read the question as "I would like to drive the FT232R with Vcc=5V and VCCIO=3.3V, will this be okay with 5V RXD/TXD?", but without being explicit that the voltage configuration is not open for discussion. Hence, I assume, the question from EugeneSh and my comment. Usually, I'd expect a question to be clear about that constraint. For example, it is an existing device (like an FTDI cable), where there is no room for change. \$\endgroup\$ – gbulmer Jan 13 '16 at 21:43
  • 1
    \$\begingroup\$ Ah, ha! That would be a great addition to your question. I can imagine lots of people might find that idea helpful, and heap kudos on you. \$\endgroup\$ – gbulmer Jan 13 '16 at 21:45
1
\$\begingroup\$

If you dig into the datasheet and infer a few things, the "Vin" parameter doesn't change no matter what level you drive VCCIO. (See Tables 5.3 - 5.10)

This means that the input circuitry is not affected by the VCCIO level, and it only cares whether you exceed its threshold level of around 1.2V.

As the comments pointed out, it seems like an odd scenario to drive VCCIO at 3.3V (making the outputs 3.3V) while taking 5V on the input pins, but there isn't a technical problem with it.

\$\endgroup\$
2
\$\begingroup\$

The datasheet does say in "Absolute Maximum Ratings",

DC Input Voltage – USBDP and USBDM: -0.5 to +3.8V
DC Input Voltage – High Impedance Bidirectional: -0.5 to + (VCC +0.5)
DC Input Voltage – All Other Inputs: -0.5 to + (VCC +0.5)

RXD/TXD should be okay driven by a 5V device, they are operating below their absolute maximum.

It is common for signals to be below 5V.

What is connected to RXD/TXD?

Voh, Output Voltage High, is only min: 2.2V, typical: 2.7V, max: 3.2V

at VCCIO=3.3V, which might have a bearing on your decision.

Edit:
Some 5V devices might like about 2.5V, so someone might get unlucky, and have a FT232R device with a minimum of 2.2V, and get unreliable results. This will likely be rare, but maybe intermittent, and hence unpleasant to debug.

I think Daniel's answer covers the input-pin case.

\$\endgroup\$
  • \$\begingroup\$ TXD (sending): Either a +5V device or below (3.3V, 3V or 2.8V) RXD (receiving): Nothing \$\endgroup\$ – d3L Jan 13 '16 at 21:41
  • \$\begingroup\$ So this would confirm my assumption: the FT232 accepts logic input levels from 1.8V to 5V (given VCC is 5V) but outputs only up to VCCIO. \$\endgroup\$ – d3L Jan 13 '16 at 21:42
  • 1
    \$\begingroup\$ Mostly agree. The small concern I have is the minimum Voh at 2.2V. Some 5V receivers might like 2.5V or more. So you might get unlucky. \$\endgroup\$ – gbulmer Jan 13 '16 at 21:48
  • \$\begingroup\$ Decent points about \$V_{oh}\$ vs \$V_{ih}\$ of the other device. \$\endgroup\$ – Daniel Jan 13 '16 at 22:36

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.