Driving 5V bus transceiver with 5V tolerant pins

[Davorin]

Good evening (o;

I know that most GPIO pins can be driven from 5V devices...

But how about connecting a 5V bus transceiver? Is that also save?

Or better to use a level shifter in between?

Reason for asking: Project involving SN75160/161 bus transceivers for GPIB interface....

thanks in advance

richard

[Dvorak.Peter]

Old school TTL uses 0.8V low and 2.4V high input logic levels.

It should work without level shifters.

For more detailed answer, just post exact schematic.

[Davorin]

Good morning (o;

No full schematic yet..but a simplified version with the chosen MCU STM32F107VCT..

[Davorin]

Seems to work that way..though only tested so far in one direction...

At least I see commands and data coming from the GPIB bus on the Nucleo-F207 USB console (o;

CMD: 40 	MTA0
CMD: 3F 	UNLISTEN
CMD: 25 	MLA5
DAT: * 
DAT: I 
DAT: D 
DAT: N 
DAT: ? 
CMD: 3F 	UNLISTEN
CMD: 20 	MLA0
CMD: 45 	MTA5

Now I only have to find an easy and elegant way how to switch certain GPIO pins from output to input...

Or can I always read the GPIO input when it is configured as GPIO open drain output with pull up?

[Dvorak.Peter]

https://www.st.com/content/ccc/resource/technical/document/application_note/group0/13/c0/f6/6c/29/3b/47/b3/DM00315319/files/DM00315319.pdf/jcr:content/translations/en.DM00315319.pdf

AN4899 Application note

STM32 GPIO configuration for hardware settings and low-power consumption

Page 17:

However, a GPIO is five-volt tolerant only in input mode.

 When the output mode is enabled, the GPIO is no more five-volt tolerant. 

For more details about I/O input voltage, refer to VIN

parameters in the general operating conditions table of the datasheet.

DocID029601 Rev 1

==============================

The above guide suggests that a IO output connected to TTL output (high level) could create over-voltage problem.

A simple fix is to use Bidirectional Voltage-Level Shifter with Auto Direction Sensing

[Davorin]

Hmmm...so far I didn't had problems with it...though I used data bus only in receiving mode...

but the GPIB control bus is mixed....so I could read GPIB control signals as well as driving control signals....otherwise the above GPIB bus capture wouldn't have be possible...

The SN75161 device is not a straight transceiver from left to right or vice versa....

so for example I tested DAV being read from MCU and NRFD/NDAC being driven by MCU....so far the MCU survived it (o;

Guess special care must be taken at which time the GPIO direction configuration and the transceiver direction change takes place...