Hi All,
I’m quite new to LPC programming, but I’ve managed to make some decent
progress using an Olimex LPC-P2138 eval board and Rowley Cross Studio.
My problem now is that my setup is very sensitive to EM interference.
When I start my oscilloscope for example, or even when I move a few
wires around, my EINT interrupt handlers fire off.
I’m only using 2 EINTs (pulled up) and UART, and the rest of the pins
are set to output.
Any ideas how I can stop this behaviour?
Thanks.
The behavior you’re seeing is not so strange. Your power supply and the scope probably are connected via AC ground, or at the least have a capacitive connection that way.
I always use a two-wire AC adapter on both the power supply and scope which helps a lot, but is no magic cure.
Keep the scope ground lead as short as possible, and once attached, leave it connected.
Thanks for that.
The board is powered by a PC power supply (5V) so I guess it’s reasonably well insulated from ground. The scope causes the board to go crazy (it’s a very old scope, so it’s probably emitting x-rays!) but I see the interrupts firing even if I shift a couple of wires near the board when the scope is off, like they were acting like antennas.
I was thinking about shielding the board in a metal box, but surely it shouldn’t be this sensitive to intereference?
Maybe it’s the Olimex design? I might try a different make.
A PC power supply is definitely not a low noise device, and will have a capacitor between its ground and earth ground, the green wire. Get a wall-wart type suppy that is more appropriate for the power you’re using.
Those wires you’re calling antennas are exactly that, and what they are picking up is probably the EMI from the power supply. Those units are designed for 100 Watts or up, and with no load on them are akin to a radio transmitter.
Thanks for your reply. THe power supply in this case is 450 Watts although it is under load since it’s running the PC at the same time. I assumed the power supply would be “clean” since it’s designed to power a motherboard.
I also tried with a plug-in type 300mA power supply and the behaviour was the same.
Could it help if I pulled all the unused pins on the board down to ground?
I appreciate your help with this.
Ben.
Ouch! You have to have some serious ground loops going on if you’re powering the device from the same PC that you’re using for the jtag interface. That’s not a good idea.
Program unsued pins to be outputs and set them high so that you don’t have to supply power to the weak pullups internal to the chip.
Hi Smead,
I tried what you suggested and I tried powering the board from batteries to see if that made any difference, but the problem is still the same. Anything going on around the board that might generate EM interference fires the EINTS.
Would shielding the board help do you think?
Thanks.
Ben.