# Forum: ARM programming with GCC/GNU tools LPC2129 doesn't start when attached to an ULN2803

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Hi!!

I'm experiencing the problem that when I connect an ULN2803 to a LPC2129
ports the MCU doesn't start, and when unplugged such a driver from its
socket, the MCU starts to work as usual.

The eight lines goes directly from P0.11 to P0.18 to each input of the
driver. If I just connect any single pin of the MCU to any single input
of the driver, the MCU doesn't start!!. I read that P0.14 is needed to
enter into debug mode (or something like that), but for example, if only
P0.11 is connected to the I0 of the driver, the MCU doesn't start, and
so do the rest of the pines. Even more, P0.11 to P0.18 had been set as
inputs, but this still doesn't work. I've placed the lines that
initialize those terminals in several parts of my code, without good
results. My reset line has a 10K resistor and a 1uF tantalium cap tied
to ground.

Moreover, when debugging under CrossWorks this works fine, but when
stand-alone this didn't.

Is there something wrong with this MCU when interfacing with such a
drivers?? Do I need to add pull-down resistors??

Thanks!!

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Javier Rod wrote:
> Hi!!
>
> I'm experiencing the problem that when I connect an ULN2803 to a LPC2129
> ports the MCU doesn't start, and when unplugged such a driver from its
> socket, the MCU starts to work as usual.
>
> The eight lines goes directly from P0.11 to P0.18 to each input of the
> driver. If I just connect any single pin of the MCU to any single input
> of the driver, the MCU doesn't start!!. I read that P0.14 is needed to
> enter into debug mode (or something like that), but for example, if only
> P0.11 is connected to the I0 of the driver, the MCU doesn't start, and
> so do the rest of the pines. Even more, P0.11 to P0.18 had been set as
> inputs, but this still doesn't work. I've placed the lines that
> initialize those terminals in several parts of my code, without good
> results. My reset line has a 10K resistor and a 1uF tantalium cap tied
> to ground.
>
> Moreover, when debugging under CrossWorks this works fine, but when
> stand-alone this didn't.
>
> Is there something wrong with this MCU when interfacing with such a
> drivers?? Do I need to add pull-down resistors??
>
> Thanks!!

Hi again!!

I guess I solved my problem. The ULN2803 input resistors tied P0.14
down, so it started the serial bootloader mode. I add a 4K7 pull-up
resistor and now it seems to works in stand-alone mode. This is a
thermal printer driver, but it doesn't want to print anything (it does
under CrossWorks), but that's another history.

Thanks for reading the post!!

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