TMS320F28075: About the connection of VSSOSC on F2807x and F2837x devices

Sheldon He

Expert 8585 points

Part Number: TMS320F28075
Other Parts Discussed in Thread: LAUNCHXL-F28379D

Hi expert,

It is a bit confusing when I meet the connection of VSSOSC on F2807x and F2837x series device. I have below schematics.

I encountered the problem that I am not able to program the flash.

e2e.ti.com/.../753199

But if I connect VSSOSC to ground, everything works fine again.

So I have following questions:

Firstly, I'd like to check if F2807x and F2837x have the same requirements on the connection of this pin?

Secondly, on F28377s and F28379D launch pad. One is not connected to ground, one is not, why? (see picture in the end)

Thirdly, I really need your help to deeply understand below lines. What do you mean by "board ground" and "ground reference of OSC circuit"? Do you mean "board ground" VSS?

Attached picture, from launch pads.

Thanks

Sheldon

over 6 years ago

0 Frank Ankapong over 6 years ago

TI__Mastermind 28275 points

Sheldon,

Yes, that should be right. By the way, your load caps of 33pF is too high. We spec a maximum of 24pF. Vssosc acts as kelvin ground. Please provide scope shots of the clock through XCLKOUT of vssosc connected to the kelvin ground of the crystal and also connected directly to the board ground.

0 Mark Labbato over 6 years ago in reply to Frank Ankapong

Guru 30330 points

Just chiming in on the Launchpad designs:

On the LAUNCHXL-F28379D the VSSOSC is not tied to ground. The "NI" indicates that the resistor is "Not Installed", It can be if a different clock source needs to be used. It does not look like we provided that capability on the LAUNCHXL-F28377S and VSSOSC can only be tied to the External oscillator ground reference.

The F2807x and F2837x devices have the same XTAL requirements.

-Mark

0 Sheldon He over 6 years ago in reply to Mark Labbato

TI__Expert 8585 points

Hi Mark,

This is the signal from X1 and X2 with VSSOSC not connected to ground. Is that okay?

BTW, will guide "Hardware Design Guidelines for TMS320F28xx and TMS320F28xxx DSCs" suited for F2807x and F2837x devices?

Thanks

Sheldon

0 Frank Ankapong over 6 years ago in reply to Sheldon He

TI__Mastermind 28275 points

Sheldon,

Please provide 2 scope shots of the XTAL through XCLKOUT, and not X1 and X2. Enable frequency and duty cycle measurement on the scope. The first scope shot should be for the case where everything works fine. The second scope shot should be for the problem case.

0 Sheldon He over 6 years ago in reply to Frank Ankapong

TI__Expert 8585 points

Hi Frank,

We found some soldering problem on thermal pad. We will do more test after we fix this problem.

Thanks
Sheldon

0 Frank Ankapong over 6 years ago in reply to Sheldon He

TI__Mastermind 28275 points

Sure Sheldon, no problem. Let us know if it's still an issue after fixing the soldering.

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TMS320F28075: About the connection of VSSOSC on F2807x and F2837x devices