CCS/TMS570LS0432: Two outputs connected to a single line

user5342390

Part Number: TMS570LS0432
Other Parts Discussed in Thread: HALCOGEN

Tool/software: Code Composer Studio

In my application with 2 TMS570LS04 Lauchboard, I would like to connect two output GIOs (one on each board) to the same line.

I thought about using the "Open drain" and "Pull-Down" option on both outputs thanks to HalCoGen, but it doesn't work... The two outputs cannot be connected to the same line.

Can anyone help me how to do that? Is it also possible to do this with two NHETs in PWM?

Thanks in advance,

Maxime

over 7 years ago

0 Sunil Oak over 7 years ago

TI__Mastermind 49120 points

Hi Maxime,

You can configure them both to be open-drain outputs. In this case, you will need to connect an external pull-up as then MCUs will each only drive a low-level.

You certainly have to be careful when using PWM outputs from two pins onto the same signal line.

Regards,
Sunil

0 user5342390 over 7 years ago in reply to Sunil Oak

Prodigy 200 points

Thanks for the answer.

Do I also have to connect the internal pull-ups ? Isn't that enough?

Or no internal pull-up and only external pull-up?

Thank you!!

0 Sunil Oak over 7 years ago in reply to user5342390

TI__Mastermind 49120 points

Yes, the internal pull-ups will also work to pull up the signal when either of the parts is driving a High level. These pulls are typically weak though, so you should check the signal transition times to make sure they are what you expect.

0 user5342390 over 7 years ago in reply to Sunil Oak

Prodigy 200 points

Unfortunately, it doesn't work for me.

I connected 2 outputs to the same line (HET18 and HET22). I set HET18 to "1" and HET22 to "0", but I always get 0 on output...

(With Open-Drain and internal pull up on HET18 and HET22)

0 Sunil Oak over 7 years ago in reply to user5342390

TI__Mastermind 49120 points

An open-drain output drives the pin Low when outputting 0, but only relies on a weak internal pull-up when outputting 1. So when one pin outputs 0 and another pin outputs 1 on the same line, the pin outputting 0 always “wins”. This is expected. What are you trying to achieve by connecting the outputs together?

0 user5342390 over 7 years ago in reply to Sunil Oak

Prodigy 200 points

I have two outputs connected to the same input from another device.

Both outputs are never activated at the same time, but I would like both to be able to be used to drive this other driver.

Concretely I would like to make a kind of "mux" between two outings.

Maybe there's a completely different better approach? We can certainly do it with a multiplexer but I wouldn't like to add ICs...

0 user5342390 over 7 years ago in reply to user5342390

Prodigy 200 points

I think I found a good solution.

Note the two outputs A and B. When I use output A, I configure the output B in input. Then when I use output B, I reconfigure output B in output and configure output A in input.

Do you think this solution can work?

as I never use both at the same time, it seems to me that it should work.

0 Sunil Oak over 7 years ago in reply to user5342390

TI__Mastermind 49120 points

You don;t even have to switch to the pin being an input pin. In an open-drain configuration, when you want one of the two outputs "disabled", just make sure you are outputting a '1' on that pin. This is essentially the same as making it an input pin with a pull-up selected.

0 user5342390 over 7 years ago in reply to Sunil Oak

Prodigy 200 points

Thank you for your answers and for your time!

What I don't understand with the Open-Drain is when I want to impose a 1 on the final output. If I assign a 1 to one of my outputs, I will disable it, and I will not impose a 1...

I think there's something I don't understand about the open-drain...

0 Sunil Oak over 7 years ago in reply to user5342390

TI__Mastermind 49120 points

When you want to "impose" a 1 on the output, the pull-up will pull the signal up to 3.3V. This will work correctly as long as you don't have any external circuit that pulls down the signal.

0 Sunil Oak over 7 years ago in reply to Sunil Oak

TI__Mastermind 49120 points

Hi Maxime,

Are you okay with the implementation now? I will mark this post as resolved and closed. Please feel free to post again if anything is still unclear.

Regards,
Sunil

0 user5342390 over 7 years ago in reply to Sunil Oak

Prodigy 200 points

Hi,

Unfortunately I didn't succeed with the Open-Drain...

But the solution by switching between input/output works so the problem is solved.

Thanks for the help!

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CCS/TMS570LS0432: Two outputs connected to a single line