Lpc2000

jpsg123 wrote:

>I have a problem where the MCU won't boot up. I discovered that if I have no power on my 
>board but with a serial cable plugged in, the serial driver drives VCC to -0.5V (max), 
>probable leak since RS-232 Rx input is negative voltage. When I power up my board, the 
>chip won't boot up. But if I kill power to my board but reapply power again before VCC 
>drops below 0V, the chip would boot fine (this is possible by the way because of some 
>large caps on my board, so VCC doesn't immediately go down to 0V).
>
>Now my question, should the MCU boot up when you power up from negative VCC as little 
>as -0.5V? Should a reset chip (I'm using TPS3836 3.3) help? In my case, it doesn't. Thanks.
>
Just a tip but have you tried a bleed resistor across the supply? 
Normally any stray voltages that manage to sneak through the substrate 
can be damped this way.

This -0.5V can be doing all kinda wierd things with the substrate of the 
micro because you'll find that elements on the chip are only isolated 
from one another by normally reversed biased pn junctions. The -0.5V is 
probably an indication that these isolation junctions are now conducting 
and clamping, oowwhhh, that'll hurt on power-up!.

There is always a trade-off though because in low-power systems you 
don't want to waste power through a bleed resistor. But for most systems 
this helps enormously and hey, it's easy to put on the board or change.

my2cents

--
Peter Jakacki