Overo: Minimal expansion board

[Marco-103]

Dear all,
I want to design my own expansion board for an overo air module. For
external communication I only need the console RS232 ( for uboot ) and the
first SPI port with one CS for communication with an STM32F103x8 ( 3.3V ). (
Of course I need the onboard WLAN module but I think I need no external
connections for it ). After study of the gumstix summit PCB I think I need
the following connections on the J1 and J4 connectors:
Power:
- VSYSTEM ( x4 )
- GND ( x4 )

For RS232 ( with logic level converter 1.8V <-> RS232 ):
- GPIO165_IR_RXD3
- GPIO166_IR_TXD3

For SPI ( with logic level converter 1.8V <-> 3.3V ):
- GPIO171_SPI1_CLK
- GPIO172_SPI1_MOSI
- GPIO173_SPI1_MISO
- GPIO175_SPI1_CS1

My questions:
- You think this is sufficient or does I miss something here ?
- Do I need some external capacitor or something else ? It seems that there
is nothing like this on the summit PCB ?
- For the 3.3V VSYSTEM power I plan to use a PRH08080W switching regulator (
http://focus.ti.com/lit/ds/symlink/pth08080w.pdf ). You think it´s possible
or will I run into problems with it ( ripple voltage ? ).

Regards,
Marco




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[Søren Steen Christensen]

Hi Marco,

> the following connections on the J1 and J4 connectors:
> Power:
> - VSYSTEM ( x4 )
> - GND ( x4 )
OK - I would put a 1uF-10uF cap at the connectors close to the power pins -
Just to be on the safe side since we don't know what's actually mounted on
the Gumstix itself before the signals hit TPS65950 :-)

> For RS232 ( with logic level converter 1.8V <-> RS232 ):
> - GPIO165_IR_RXD3
> - GPIO166_IR_TXD3
>
> For SPI ( with logic level converter 1.8V <-> 3.3V ):
> - GPIO171_SPI1_CLK
> - GPIO172_SPI1_MOSI
> - GPIO173_SPI1_MISO
> - GPIO175_SPI1_CS1
>
> My questions:
> - You think this is sufficient or does I miss something here ?

I think the above should be OK - I would though add the following as well:
1) Connect AGND (J1 - pin 50) to GND
2) Connect VBACKUP (J1 pin 52) to GND in case it isn't used for at Backup
Battery
   http://focus.ti.com.cn/cn/lit/an/swca073a/swca073a.pdf tells to do so :-)
I know this isn't done at Summit, so it might not be critical though more
"correct" I think.

> - Do I need some external capacitor or something else ?
> It seems that there is nothing like this on the summit PCB ?
Agree - To be on the safe side I would prepare for caps for the power as
stated above :-)

> - For the 3.3V VSYSTEM power I plan to use a PRH08080W switching
> regulator (
> http://focus.ti.com/lit/ds/symlink/pth08080w.pdf ). You think it´s
> possible or will I run into problems with it ( ripple voltage ? ).
I see no reason why this wouldn't work - 30mV ripple shouldn't be a problem
- Just make sure it never goes below 3.2V, since you might then get into
trouble powering up. Then first TPS65950 is ON you can go (from an OMAP
point of view) as low as 2.7V-2.8V. I don't know if this causes problems
with the Wi2Wi chip though...

Best regards - Good luck - Please let us know about the result
  Søren

---
SSC Solutions ApS - Denmark - www.ssc-solutions.dk


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