7 years, 6 months ago.

Pin connection on LPC1768 if using FTDI cable

I am presently working on a projet using the LPC1768. I'm making my PCB and I was doing some last minut verifications when a question poped up. I am using a FTDI TTL-232R-3V3 (http://www.digikey.com/product-detail/en/TTL-232R-3V3-2MM/768-1095-ND/2507857) cable to program my LPC1768 and i was wondering if i needed to do anything with the JTAG pins (pull-up or pull-down) or if i could simply leave them floating. I used this (http://mbed.org/media/uploads/chris/lpc1768-refdesign-schematic.pdf) schématic as reference. Am i right to think that if i am using ISP programming i don't need to care about the JTAG pins?

this is what i have right now /media/uploads/1027912/exempleuc.png

Thanks for your time

Julien Duchesne

3 Answers

7 years, 6 months ago.

It will be safer to use some pull-up or pull-down resistors on the JTAG pins even if you dont plan to use them. The reference schematic is a good wiring example. Here is another one:


Accepted Answer

Thank you very much Wim. I dot have lots of space left on my PCB, could i juste put one resistor to 3.3V and connect everything to that resistor?

posted by Julien Duchesne 17 Jul 2013
7 years, 6 months ago.

If you connect:
DTR to Reset

with a 1 - 5 K resistor, then Flash Magic can fully program the LPC1768 using this circuit:


(from Flash Magic documentation.)

or two small transistors, with appropriate resistors.
in Open Collector mode.

Hope that helps.


Hello I have a question about what is in LPC1768-refdesign-schematic. There, in 3rd page that the schematic of Ethernet Phy is mentioned, value of resistors are mentioned by something like "2k2" or "49R9". What does it mean?

posted by Ghasem Pasandi 19 Apr 2015
7 years, 6 months ago.

Thank you Ceri! =)

But... i think that justed mixed me up un bit more because i am using a câble that already has a RS232 transceiver. I read this page (http://mbed.org/users/chris/notebook/prototype-to-hardware/) and read almost all the comments and i can up with this /media/uploads/1027912/exemple2uc.png

Could you tell if this will work? I am almost certain but would only like a confirmation! The usb to TTL 232 serial cable is this one (http://www.digikey.com/product-detail/en/TTL-232R-3V3-2MM/768-1095-ND/2507857).

Thanks for your time =)


Yes it will work, But be warned, the VCC from the FTDI cable is 5 Volts

This will KILL LPC1768 chip !!!!

I only suggested the above circuit, because it is not always possible to press switches & program,
or am i lazy

posted by ceri clatworthy 18 Jul 2013

Ceri, you are correct that a classic FTDI cable with 5V Vcc will kill the LPC1768 unless you use a voltage regulator like the 1117-3V3. The regulator would be a good idea anyhow because you can then also use USB as powersupply.

The circuit that you propose for automated activation of the ISP bootmode should work fine. The problem with the standard FTDI cable is that it does not provide the DTR pin. You only get RTS and CTS.

I have modified a cheap (actually I got it for free...) mobilephone datacable which uses the PL2303 to provide both 3V3 power and RTS/DTR. It may be possible to adapt an FTDI cable/circuit in the same way depending on your hardware.

posted by Wim Huiskamp 18 Jul 2013

Ok thank you guys! Very helpful! Don't worry about the 5 Volts, my FTDI cable is 3.3V ;). One other little thing. Why are the RTS, DTR, CTS keep comming up in every article... I don't understand if i really need them!

Best regards


posted by Julien Duchesne 18 Jul 2013

If you are happy to use switches, then you do not need RTS, DTR, CTS, these are the additional lines some programmers use, for automation. you can happily get away with manula reset & ISP (only requires to be low, while reset goes from low to Hi)



posted by ceri clatworthy 19 Jul 2013