Lpc2000

----- Original Message ----- 
From: "ds19968902" <eherna12@...>
To: <lpc2000@yahoogroups.com>
Sent: Tuesday, January 25, 2005 4:15 AM
Subject: [lpc2000] lpc2106 Overheating


>
>
> I recently posted about sourcing the lpc2106 due to burning a few of
> them lately. Leon Heller asked as to how they got fried so I put some
> information together. Now this has occured with the lpc2106 board
> from olimex, both the 40pin header and proto board. Im using
> Crossworks.
>
> The wiggler-compatible JTAG connector (Olimex) seems to be causing
> sporadic mishaps, which in turn fry the LPC2106. The normal 3.3V
> supply current to the JTAG connector itself is around .7mA, going as
> high as 1.7mA when downloading a program to the '2106. When the fault
> condition is present, however, the supply current increases to 37mA.
> I set up the JTAG connector to be supplied by by a sourcing-only
> power supply (op-amp with a transistor instead of pulling a wire from
> the headerboard since I thought it was sloppy to wire to the supply
> on the 40-pin header), and it appears as though the 3.3V supply
> current is being sent from the supply through the JTAG connector, to
> the LPC2106 straight to ground, since the supply's transistor always
> becomes hot when the '2106 becomes hot. The only way the 3.3V supply
> and the '2106 are connected is through the JTAG connector. I am
> speculating that an output on the JTAG connector is trying to pull a
> pin high that leads to the '2106, which in turn is trying to pull
> that same pin to ground. I suspect that this might be a flaw in the
> JTAG connector itself, since nothing but the JTAG is connected to the
> processor. I don't know which pin on the JTAG cable might be at fault
> here, and I cannot reliably produce the fault condition that fries
> these chips. On occasion, some action on the software side, such as
> disconnecting or processor reset causes the fault to occu

It sounds like they are going into 'latchup', the device behaves like an SCR 
and draws *lots* of current. It used to happen sometimes with the Inmos 
transputer if one touched the top of the device whilst it was powered up. 
The metal lid wasn't grounded and static could initiate the condition. They 
always recovered if the power was removed. I've occasionally had it happen 
with other devices, but it's quite rare these days, with modern CMOS.

ADI has a good description of the phenomenon and how to avoid it:

http://www.analog.com/library/analogDialogue/archives/35-05/latchup/

Leon 



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