Reset Pin problem on MSP430BT5190

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Posted by BuffaloEngineer

on Apr 13 2012 17:00 PM

Having a strange problem with the reset pin. If I short the pin to GND (using a wire), the device resets.

So I hooked it up to a pushbutton with a pull up resistor. When I push the button, I confirmed that the voltage on the pin goes from 3.3V (Vcc) to 0V, and back to 3.3V when the button is released. The MCU appears to shut down, but will not restart until the wire is removed. I'm not sure why since it has returned to 3.3V.

Both SYSRSTUP and SYSRSTRE are set to 1.

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Posted by Jens-Michael Gross

on Apr 16 2012 10:55 AM

Guru140085 points

Did you double-check the pullup resistor? A pullup of 100Ohm or even less will alow the CPU to start on power-up, but if you shortcut it to GND with the pushbutton, it will drain VCC significanly, perhaps bringing it down. So when you release the button, it may be that the CPU crashes when it tries to restart because of instable VCC.

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Posted by BuffaloEngineer

on Apr 17 2012 13:44 PM

Intellectual490 points

I double checked the pullup resistor and tried replacing it with a 10k to see if a lower resistance value would help it recover faster. The reset pin is brought to GND for as long as the user pushes the reset button, but it recovers quickly after the button is released. Is the answer to keep dropping the pullup resistor value until it starts on it's own?

In the condition that the battery is fully discharged and the wire is connecting the button to the reset pin, the device will not start when it is plugged in.

Posted by Jens-Michael Gross

on Apr 18 2012 11:06 AM

Guru140085 points

BuffaloEngineer
tried replacing it with a 10k to see if a lower resistance value would help it recover faster.

No, my suspicion was that a lower pullup (100Ohms instead of 10k or so) would drain the battrery when teh button is pushed, and the battery needs to recover.

BuffaloEngineer
In the condition that the battery is fully discharged and the wire is connecting the button to the reset pin, the device will not start when it is plugged in.

And it does start if the wire is not connected? That's rather strange.
Normally, if supply voltage is slowly rising, MSP migh tnot properly start. For this reason, a capacitor is placed between RST and GND in addition to the pullup. (and on newer MSPs, there is an internal ~50k pullup anyway that is active on power-up). This capacitor helps preventing a crash-on-power-up, as it keeps RST lower than VCC while VCC is still rising. It would also debounce your pushbutton.
If you're using SBW interface fo rprogramming, the capacitor should be connected to RST with a 1K resistor in series, so it won't act as low-pass filter on the SBW signal. 10 to 100nF are fine.