Power leakage into MSP430 via UART Pins

hwcollector

Other Parts Discussed in Thread: MSP430F2272, TXB0302

I am using the MSP430F2272 and using a USB/UART adapter to connect a PC to the MSP430F2272.

I notice that with the USB/UART adapter plugged in to a USB port on the PC, as soon as I connect the UART side of the USB/UART adapter to the UART pins(TxD, RxD, Gnd) of the MSP430, the MSP430 starts running code. I find that this leakage of power sometimes results in an improper reset of the MSP when power is applied to the MSP.

Has anyone experienced this? Is there a simple way of avoiding this power leakage into the MSP430?

Regards,

Robert

over 13 years ago

0 old_cow_yellow over 13 years ago

Guru 58965 points

Yes. That is the curse of a low power device.

The weak pull-up resistor on the nRST pin can leak enough current to MSP430 too!

0 hwcollector over 13 years ago in reply to old_cow_yellow

Prodigy 90 points

I have an external 49k pull-up connected to Vcc and a 1nF connected to Gnd (from nRST). As power is not being applied to the MSP430 yet, I don't think this is an issue?

0 Jens-Michael Gross over 13 years ago in reply to hwcollector

Guru 227245 points

hwcollector said:
As power is not being applied to the MSP430 yet, I don't think this is an issue?

I fno power is applied to the MSP, VCC is 0V. An input voltage of 3V on the RX pin (or any othe rpin) is therefore 3V above VCC and clamped through the clamp diodes to VCC, resulting in 2.8V supply voltage applied to VCC. The clamp diodes can sink up to 2mA before breaking (sometimes shorting the pin to VCC, sometimes leaving it unprotected). However, 2mA (or even much less) are enough to make the MSP start.

If you have signals that are powered before the MSP, you must ensure that they are either disconnected as long as the MSP is not powered, or you must keep the clamp current very low, e.g. by a large series resistor.

0 hwcollector over 13 years ago in reply to Jens-Michael Gross

Prodigy 90 points

Hi Jens-Michael,

Thank you for your answer, that makes sense. Do you have any suggestion on a suitable resistance value to use?

0 Peter Dvorak over 13 years ago

Expert 2740 points

hwcollector said:
Is there a simple way of avoiding this power leakage into the MSP430?

You can use 2-Bit Bidirectional Voltage-Level Translator with Automatic Direction Sensing txb0302. A bit of an overkill but should do the trick.

Peter

0 hwcollector over 13 years ago in reply to Peter Dvorak

Prodigy 90 points

Thanks for your suggestion Peter, it is an excellent one as I do need a level Translator in one of my applications.

**Attention** This is a public forum

MSP low-power microcontrollers

MSP low-power microcontroller forum

Power leakage into MSP430 via UART Pins