PCM2706C: SSPND pin never low, even when device is idle

Gerfried Essler

Part Number: PCM2706C
Other Parts Discussed in Thread: PCM2706

Hello!

I have build a prototype similar to figure 35 in the datasheet. (bus powered mode with some subsequent circuitry).

I want to achieve that this subsequent circuitry is turned off when no audio is processed by the PCM2706 using the SSPND signal.

The problem is, that the SSPND pin never goes to low, even when there are no data transmitted on D+/D-.

I don't know much about the USB protocol but I scoped the lines and saw a short "blip" every few ms on the bus.

Can this be some kind of keep-alive signal that keeps the PCM from going to suspend?

Greetings,

Gerfried

over 5 years ago

0 Paul_Frost over 5 years ago

TI__Guru 53183 points

Hi Gerfried,

I will look into this and get back to you after the US holiday - likely Monday.

Thanks,
Paul

0 Gerfried Essler over 5 years ago in reply to Paul_Frost

Prodigy 40 points

Thank you for the quick reply :)

I have checked the D+/D- pins again with my scope and saw that the "blips" are once every millisecond, if that helps.

I have no logic analyzer so I don't know what data is transfered there

ps: This is how the data on D+ looks like:

0 Paul_Frost over 5 years ago in reply to Gerfried Essler

TI__Guru 53183 points

HI Gerfried,

Yes, I believe these regular communications are preventing the device from suspending. Unfortunately I do not know much about the USB protocol - is this a feature of your operation system?

0 Gerfried Essler over 5 years ago in reply to Paul_Frost

Prodigy 40 points

I observed this same signal when I connected my PCM device to a laptop with Linux as OS and a cell phone with Android. Maybe Windows or Mac doesn't send these signals? I haven't had time to test these two OSs. Strange though. Was the PCM2706 tested with another USB driver/version?

0 Paul_Frost over 5 years ago in reply to Gerfried Essler

TI__Guru 53183 points

Hi Gerfried,

This is really more of a USB question, as I think the OS is preventing the devices from entering a suspend mode. Check out this article:
www.kernel.org/.../power-management.html

I think 1ms idle frames are expected if the OS is preventing suspend mode.

Thanks,
Paul

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PCM2706C: SSPND pin never low, even when device is idle