Hi!

I have a PCB that (among other electronics) contains a EM250 ZigBee module connected over the UART interface with a FT232 FTDI IC, as shown in the "ftdi_mcu-current.gif" attached figure.

The problem is that due to this connection the power consumption of the PCB is higher than I expected. The FTDI is power from the USB bus (and not from the power supply of the PCB) which is however not all the time present. Thus, I assume that the high power consumption that is measured only when the USB supply power is not present comes from current throwing from/to the pins of the EM250 to/from the pins of the FT232. I have tried all possible states of the pins of the uC but could not go too low in the power consumption (the best is around 700uA, while around 200uA was expected).

I have removed the FTDI completely and then the consumption is low. Thus, something is wrong with the interface between the uC and the FTDI.

To reduce the power consumption further, I was thinking of making the modification shown in the "ftdi_mcu-new.gif" attached figure, thus adding some buffers between the two ICs that are going to be activated only when communication is required (when the USB supply power is actually present) and remain deactivated otherwise.

Do you think that this will work? Do you have any other alternative to propose me?

nikos

 

Last time I was playing with this FTDI i noticed it still gets powered probably through the protection diodes on the i/o pins. You could try adding some resistors in series with the data lines.

 

Last time I was playing with this FTDI i noticed it still gets powered probably through the protection diodes on the i/o pins. You could try adding some resistors in series with the data lines.

That's interesting... you mean to intercept resistors in series between the FTDI and the uC? At the TXD and RXD lines? How would that help the situation?

 

Not sure, but that is what my teacher to do to solve this problem next time, though I never actually tried it.

 

Well, just to update you. I've solved the issue.

The secret was that during sleep, the internal UART module of the uC should be deactivated and the relevant pins should be configured at input direction with no pullup/pulldown.

That was all!

Regards, Nikos