AM335x Slow Slew Rate

I have exactly the same question - where is the list of peripherals that support slow slew rates? 

Also it isn't clear what the different slew rates actually are (all the TRM says is a vague "fast" and "slow"). 

Can someone at TI clarify?

Thanks.

The AM335x data sheet has the following statement 'The timing parameter values specified in this data manual assume the SLEWCTRL bit in each pad control register is configured for fast mode (0b).'  

This statement applies to all peripherals.

Regards,
Paul

I saw that statement.  However, I cannot find any reference to the impact on slew rate of changing the SLEWCTRL bit to the slow mode setting. 

Or is it implying that all peripherals are fixed at the fast rate (i.e. no peripherals support the slow setting)?

Thanks.

This statement is saying, the timing parameters provided in the data sheet were validated with slew rate set to fast and may not be valid when set to slow.  Since the device was not validated for slow slew rate, data related to this setting is not provided in the data sheet.  So yes, it is implying you should only use fast slew rate.

Regards,
Paul

Dear,

Is there more info available on what modules do support SLOW slew rate control ?

For EMC reasone I wanted to put the slew rate for the LCD data lines to slow but I can't see any differences in signal, is it possible that the LCD modules doesn't support  slow slew rate ?

AM335x

Regards,

Steve

Hi Paul,

Thanks for the clarification. Even if it hasn't been validated, it would still be useful to know what difference slow mode is supposed to make and which peripherals/pins (if any) support it.  Is that data available?

Thanks.

Slew rate control is a function implemented in the IO cell of the device and does not have any relationship to specific peripherals.  So slew rate control is applied on a per terminal basis via the respective conf_<module>_<pin>_slewctrl bit, where '<module>_<pin>' is the terminal name.

We also determined the IO cells used in AM335x were not providing any significant change in slew rate. Therefore, there wasn't any value in characterizing the device while operating in slow mode.

We do not recommend using slow mode and there is no data for slow mode because of the reasons mentioned above.

Regards,
Paul

Thanks for that information Paul, it confirms my measurements.

Do you have any tips on reducing the EMC emission on the LCD lines maybe, filters?

Regards,

Steve

I have seen other customers add filters which I assume were inserted to reduce radiated emission.

The LCD should be one of the easier peripherals to filter since all signals are propagating from AM335x to the display.  If you use the same filter design on each signal, the insertion delay through the filter should be very similar which allows all signals to retain their timing relationship.  However, you may cause signal integrity issues if you slow down the edges too much relative to the fastest signal toggle rate required by the LCD interface.

This is a system level issue which has multiple dependencies that should be considered to determine the best solution for your product.

Regards,
Paul

Thanks Paul, I'l do a search on these type of filter, a serie resistor didn't work.

You may want to use Spread Spectrum Clocking on the LCD lines.

See appended code fragment. Use it in your board file.

regards

Wolfgang

I’ve looked at Spread Spectrum but the energy is still emitted only spread in a more width frequency band.

I need to keep the EMI as low as possible for the reception of GPS signals.

For EMI filtering of LCD lines, I use "T" filter with 2 ferrite beads and 1 capacitor per line.

For LCD clocks up to 10 MHz, I use ferrite beads with 220 Ohm @ 100MHz, and a 100pF capacitor.

For LCD clocks up to 30 MHz, I reduce the capacitor to 33pF.

For the ferrite beads, I am using arrays with 4 beads. Taiyo Yuden BK20104W241T.

The "4W" and "HW" series from Taiyo Yuden are very good because of the reduced ringing.

Our LCD is running on 36MHz, so the second option is possible, space is the big problem on out design so would preffer an integrated package.

I was loocking at TDK EMI filters MEA2012Lxxx available on 50, 100 and 200 ohm, the cap is around 34pF.

Will give you an update of the results.

Thanks for the respons Wofgang.

Dear. Steve Deschryver

Could you tell me about effect of TDK EMI filters.

I'm faced with similar your problem.

Hi Daehun Song,

 We got very good results with the filter added (LCD + high speed SDIO).

 + EMC measurements are better for the LCD Clock.

 - On GPS receiver we don't really see any big difference, still working on this now.

Regards,

Steve