On Fri, 25 Feb 2005 07:54:19 -0800, Gordon Kruberg <gordon@...> wrote:
> I have tried to take all points into consideration and I would like to hold
> a new design out for further comment. Here:
Looks much better.
> I revised the board to use 2mm spacing on all connectors except the servos,
> uarts, and I2C. Servos seem to universally use the 0.100" spacing.
> Can I do 0.100" board that's bigger? Yes-- but is 2.00mm pitch OK (versus
> 2.54mm == 0.100")?
I think that 2mm pitch is fine.
> I have also borought out power and ground adjacent to all signals: AGND and
> AVCC to the ADC pins, VCC5 to the UARTs, V_MOTOR and GND to the servos, and
> V_BATT to the remaining power pins. I would especially appreciate comment
> on this as it pertains to the overall power strategy; namely, that V_BATT is
> provided as a power source bus, reasonably regulated at 5V and providing 1A
> total. Vcc is 3.3 and is intended for this board only; Vcc5 similarly.
> The UART arrangement is a little different, but allows power and ground as
> well as the chance to jumper level-shifted PXA signals to the ATmega128.
> The SPI bus is strictly between the PXA and ATmega128 for communication and
> boot-download at this point, with the PXA as the master and thus limited to
> 3.6864MHz per the intel docs.
I'm confused by your power stuff. If I'm reading your comments above
V_BATT = 5v
Vcc5 = 5v
Vcc3.3 = 3.3v
However, on the schematic you show Vcc5 as coming out of a MIC5255,
which doesn't appear to make a 5v device, and you can't put 5v in and
get 5v out anyways.
It's important that the voltage present on the connectors be the same
as the voltage that the ATMega128 is being driven with (i.e. V_BATT =
Vcc5) (the whole purpose of having the +5 on the conenctor is so that
it could be connected through a sensor to the signal pin).
So if there's some onboard voltage regulator that's taking the power
jack and going down to 5v, then the power jack voltage CAN'T be put on
the connectors. If you're assuming that the power jack is providing 5v
regulated, then you wouldn't need an onboard 5v regulator, just a 3.3v
one for the level shifters.
> Have I missed anything major?
I still think you're missing the pullup resistors on the X_SCL and
X_SDA signals. The PCA9517 data sheet shows that pullups are needed on
The schematic shows X_MISO connecting to ATM_MISO and X_MOSI
connecting to ATM_MOSI, shouldn't they cross over? i.e. X_MISO to
ATM_MOSI and X_MOSI to ATM_MISO.
> Finally, you may note that there are still a few lines open. Any
> selections? Oh, and I think that I will bring out the PA onto unpopulated
> pads on the reverse side.
It would be good to have at least one output pin (say PE2) from the
ATMega go through the voltage shifter and connect to an input GPIO pin
on the gumstix, so that the ATMega could grab the attention of the
gumstix (by configuring the gumstix GPIO to cause an interrupt). Since
SPI is driven by the master (i.e. the gumstix) it can easily interrupt
It would also be good to take one I/O pin on the ATMega (say PD5) and
connect it to an LED on the board (useful for a heartbeat).
I see that there are 3 extra pins on the AT_INTS connector. If you
don't get anything else for these, you could always connect ALE, RD,
WR to them since they're also available as Port G pins.
Having PA0 thru PA7 brought out to pads on the back would be nice.
It would be good if the pins required to use the USB connection were
available so the circuitry could be added if desired (this would be
part of the pads on the back you were talking about).
I'd like to propose a slightly different layout for the AT_UARTS. The
way it's currently done, you'd need to find 2x2 1.25mm pitch
connectors. If such connectors are readily available, then I'm happy
with the current layout, otherwise, I'd propose using a layout where
each connector is a 1x4 rather than a 2x2.
It would also be nice if it used the same 2mm pitch as the rest of the
connections (I was able to find 2x2 connectors at 2mm pitch).
The 1x4 layout would be something like this:
which still allows the serial connectors to all be wired the same and
allows jumpering for IR_TX/RX <=> ARM_RX/TX0
> Thanks for the comments,
Happy to oblige :)
Vancouver, BC, Canada