11 years ago.

Questions on making a custom LPC1768 PCB following an mbed development

We have a working prototype of a new product running on the mbed NXP LPC1768. Development has gone well. We would now like to make a custom LPC1768 board to productionise the product but I have a few questions that the forum might be able to help with.

- is the correct part number for an LQFP device LPC1768FBD100,551?

- we don't use ethernet, but we do use USB to load user files into our firmware (pins 32 & 32). Will this work correctly when the mbed special interface device is not used? Is the TGT_SCON programmable pull-up transistor required?

- without the interface chip, how would our programme be loaded?

- without the interface chip are there any special considerations about the LPC1768 reset to take account of?

- are there any other pitfalls that I should look out for?

Thanks very much for any help - sorry if these questions seem naive, but it's so easy to get to an advanced point with mbed without a good understanding of exactly what is happening in certain areas!

Mark Leese


3 Answers

11 years ago.

2) The TGT_SCON transistor is needed when you want to use USB in device mode.

3) without the interface chip you can program the lpc1768 using JTAG, SWD or through serial port UART0. The other option is USB bootloader on pin31/32 but that requires that your binary is mapped at a different address from what the mbed compiler normally provides.

Accepted Answer

Hi Wim, thanks very much for that - very useful, feeling confident about the job.


posted by Ross Whittle 30 May 2013
11 years ago.

Have you seen: http://mbed.org/users/chris/notebook/prototype-to-hardware/ ?

1. It is LQFP and LPC1768, so guess it is correct.

2. USB on those pins would still work the same. Do take into account there is no localfilesystem without the mbed interface chip. No idea regarding that transistor, I would check reference designs for the LPC1768/similar ICs.

3. See the link

4. No idea, but I don't expect much special stuff regarding a reset.

Hi Erik, really appreciate your help, sorry I didn't find the prototype-to-hardware page earlier. Many thanks for your assistance,


posted by Ross Whittle 30 May 2013
11 years ago.

Hi Mark,

1) Yes!

2) As other have written The TGT_SCON transistor is needed when you want to use USB in device mode. To run as a host, you need 15k pull downs on D+ and D-

3) As other have written, JTAG/SWD is one option, the other is Serial Bootloading on UART0. The note book page http://mbed.org/users/chris/notebook/prototype-to-hardware/ covers this

4) Reset circuit of LPC1768 is most generically made with an 10k pull-up and a 100nF capacitor to ground. An optional reset push button across the capacitor.

5) A few other considerations, mostly standard stuff

Ensure the "ISP" pin has a pull up (10k), so that the LPC doesn't enter ISP mode

If you are using a crystal rather than an oscillator, make sure you specify the right value capacitors for the chosen crystal (I've been burned by this in the past)

Ensure your PCB and some convenient test points for power, ground, reset, clock, and other bits that are important to your design

Don't scrimp on decoupling capacitors! I usually use one per Vdd pin.

Keep your USB trace lengths reasonably well matched. Use power planes if possible, if you;re keeping a 2 layer design, flood fill top and bottom as the best approximation to Keep high speed digital away from analog.

Hope this helps!

Thanks, Chris

Hi Chris, great stuff! All the guidance I could need. As I said to Erik, I'm only sorry I didn't find the prototype-to-hardware page earlier. Feeling good about the project: very many thanks,


posted by Ross Whittle 30 May 2013