Balloon 2 Mods

For PCB 2.054 (and 2.052).

U24 USB power mod

There is a design flaw on all Ballon 2.05 boards (and maybe earlier ones too) which means that, if the USB chip is fitted, the data bus can be corrupted before the USB power is set explictly on or off.

The USB chip is on the data bus and has its power (VDD_3V_USB) controlled by the high-side switch chip U24 (from GPIO25 on the CPU).

When the power to the USB chip is off but the databus is being driven 'bad karma' can happen as the chip tries to power itself off the databus. It doesn't quite manage this and the net result is that the USB power and clock cycles up and down at about 1Hz and bad data can get on the bus.

This isn't always fatal, even on boards that don't float in the right state - sometimes the CPU gets lucky and reads the right data from NOR anyway. On most boards the U24 VDD_3V_USB output seems to stay low anyway, so the problem is not noticed.

Once the USB power is explictily turned on (or off) then the problem goes away, but during early-stage bootloader activity there is the risk of curruption.

The fix is to tie a 10K pulldown to 3V_USB_OFF so that the USB power defaults to on when it is not driven. This is most easily done with a 1/4W leaded resistor between the rest switch GND and pin 4 of U24. A slightly prettier job could be done with an SM resistor and some mod wire.

We recommend this mod is performed on all boards in the interests of reliability.

http://balloonboard.org/hardware/205g/U24moddetail.jpg

Balloon 3 Mods

Some to come, no doubt.

Balloonboard: HardwareMods (last edited 2009-02-22 21:00:15 by localhost)