MAX3232 with several microcontrollers

Joaquin Gonzalez

Other Parts Discussed in Thread: MAX3232, TMS320F28335, SN74LVC1G125

Hi to everybody,

I have to connect the Rout pin of a MAX3232 device to the UARTRX pin of 3 different TMS320f28335 and I am not sure if a direct connection will work properly or if I would have to connect a buffer in between.

What do you guys suggest?

Thanks in advance,

Nubba.

over 13 years ago

0 Ron Michallick over 13 years ago

TI__Guru**** 165326 points

Nubba,

The TMS320f28335 input current is less than 2uA max and 2pF typical. No buffer is needed.

Regards,
Ron Michallick

0 Joaquin Gonzalez over 13 years ago in reply to Ron Michallick

Expert 1200 points

Thanks a lot Ron,

Due to other requirements of the pcb, now I think that i have to buffer the ROUT output with a SN74LVC1G125.

SN74LVC1G125 has an input current of 5uA and a input capacitance of 4 pf. I don't think that connecting ROUT pin to 3 of these devices at the same time would be a problem due to its parameters. Am i right?

On the other hand, do you consider SN74LVC1G125 a situable device for driving a UART? I'll use a damping resistor of 33 ohm.

Thanks in advance.

0 Ron Michallick over 13 years ago in reply to Joaquin Gonzalez

TI__Guru**** 165326 points

Nuba,

I attached curves showing the typical ROUT drive capability.
Why is this ROUT signal a concern? Are the UART pins very far from the MAX3232?
RS232 signal are low frequency signals, I do not expect any problems.

Regards,
Ron Michallick

MAX3232 ROUT Sweeps.xls

0 Joaquin Gonzalez over 13 years ago in reply to Ron Michallick

Expert 1200 points

ROUT pin has to go to three different microcontrollers at the same time, as said before. They are not far from MAX3232, but they have different power domain (3.3V from 3 different batteries).

I think that connecting ROUT to three SN74LVC1G125 devices at the same time, each of them supplied by the corresponding batery, will work better and would limit the damage in case of a failure in one of the micros.

0 Chris Cockrill over 13 years ago in reply to Joaquin Gonzalez

TI__Mastermind 46480 points

This should work just fine. The LVC1G125 can accept up to 5V input at any valid Vcc. So when any of the batteries get low it will still function properly.

0 Thomas Kugelstadt over 13 years ago in reply to Joaquin Gonzalez

TI__Mastermind 28265 points

Nuba,

my concern would be what happens if one of the three batteries goes too low and the LVC125 buffer stops operating (or get unpowered). Depending on the internal design of the buffer inputs, these might not stay high-impedance. I recommend checking the input impedance of the buffer at a lower voltage than the minimum supply specified in the data sheet.

If the internall connection of unbiased FETs becomes too low-impedance than you might need some high value resistors between ROUT and each buffer input.

So these are my 2 cents of opinion.

regards, Thomas

0 Joaquin Gonzalez over 13 years ago in reply to Thomas Kugelstadt

Expert 1200 points

Hi Thomas,

That is a good point.

In the datasheet it is said that max. current at A or OE input is 5 uA for Vcc going from 0V to 5.5V, so I guess that the imput remains high impedance when is unpowered. Im I right?

Thanks in advance.

0 Chris Cockrill over 13 years ago in reply to Thomas Kugelstadt

TI__Mastermind 46480 points

The LVC125 will function properly down to 1.65V. if these are 3.3V bat powered devices i suspect they will have some type of supervisor the turns it off at some point.

If the vcc on the LVC is 0V then thei inputs and outputs will be high impedance. Between 1.5V and about .5V the part may not meet leakage specs.

If they would like to narrow this window they could use the SN74AUP1G125 which will function down to 0.8V Vcc.

0 Thomas Kugelstadt over 13 years ago in reply to Joaquin Gonzalez

TI__Mastermind 28265 points

Hi Nuba,

theoretically you are right. I just recommend checking theevice bahavior in practise prior to PCB design. Better safe than sorry.

I might sound paranoid but that is what 34 years in electronics application and design can do you.

Regards,

Thomas

0 Thomas Kugelstadt over 13 years ago in reply to Thomas Kugelstadt

TI__Mastermind 28265 points

Nuba,

sorry, I saw Chris' response too late. If he confirms the high-impedance inputs at 0V supply that's good enough.

Thomas - Out

0 Joaquin Gonzalez over 13 years ago in reply to Thomas Kugelstadt

Expert 1200 points

Hi to all,

Although it is a battery powered system, 3.3V is regulated. The regulator has an undervoltage lockout that will disable its output in an undervoltage condition.

So I guess it should work.

Thanks a lot to all of you.

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MAX3232 with several microcontrollers