Design Recommendations of Custom Expansion Board for Overo Series

´╗┐To help improve the design of a custom expansion board for the Overo series, this list offers hints and tips with some potential design pitfalls to be avoided:

  1. Use the SYSEN line to protect any IO pins to the OMAP CPU. SYSEN is brought high when the Overo is ready to communicate; driving GPIOs before this point can damage the processor.

  2. Like SYSEN, USB_CPEN can be used as an enable line for USB_VBUS; this is done on several Gumstix expansion boards such as Tobi. However, it is not necessary to gate the USB_VBUS line; as long as VBUS on the USB connector gets 5V, the USB interface should work correctly.

  3. For high-speed signal lines, consider adding small-valued series resistors to limit signal reflections. Users have reported success adding 33 ohm series resistors to the CLK and CMD lines of the MMC3 external interface.

  4. Noise on the UART_RX3 line during boot can interrupt the normal booting sequence. Consider adding a pull-up resistor to a SYSEN-gated 1.8V supply on this line.

  5. VBACKUP provides battery backup to the real-time clock. It should be left open or connected to a rechargeable lithium coin cell which is trickle charged when the Overo is powered on. Do not tie this pin to ground as suggested in the PMIC documentation without disabling the trickle charge feature in software.

  6. ADC inputs should not be driven above 2.5V.

  7. Expansion board PCB should be less than 1.2mm thick to accept Overo retaining spacers 48 / 48. Hole diameter should be 1.65mm.

  8. Expansion board PCB should be less than 1.55mm thick to accept Overo retaining spacers 48 / 62. Hole diameter should be 1.65mm.

  9. Ensure a good connection of the Overo COM onto the expansion board via the pair of 70-pin connectors. An example of making a good connection can be found in the Gumstix videos posted on Youtube. eg Connecting an LCD and Overo COM to a Palo43 board.

  10. The design should allow flat, horizontal connection of the flex cable into the 27-pin connector (J5) both for the process of flex cable insertion and during operation.  

  11. Both the flex cable and the 27-pin connector (J5) should be cycled a maximum of ten (10) times. Insertion and removal of a flex cable would be considered one (1) cycle.

Other key links for designing with Gumstix.

Last modified May 15 2002: adding recommendations regarding flex cable and 27-pin connector.

Modified April 17th 2012: added reference to Overo retaining spacers 48 / 62

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