Let's Make Robots!

USB<>FTDI ..... Hacking a £2 module to bring out DTR signal for Boot-Loader


Here is a simple Hack to use a USB<>FTDI module for mcu programming.

Most MCU chips require a reset signal to Shout the "Boot-Loader" into action and download code.

This one i am using on a really bare bone propellor board.....

Here is my £2 module :-

Please note and be careful to check the "RESET" pin when buying these cheap modules

......as this is not the reset line that goes to your MCU, this is often a trap easily fallen into.

You need to bring out the DTR signal.... some modules have this presented but not connected.

I am using the CP2102 chip set and the DTR is Pin 28

All you need to do is solder a small wire to this and attach a 0.1uf capacitor (104 type) from it to a spare pin.

I choose to use the original Reset pin of the module which required a track cut.

Testing ......... on my beloved Propellor board ..... Bingo

 

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I have had success with using the signal going from TX on the adapter, coupled through a simple diode and a 3.3nF cap to the reset pin on an AVR, which again is pulled high with a 10k ohm resistor.

  • (TX) -----|<|-----||----- (reset) -----10k----- (Vcc)

The TX pin is high on idle. When the first serial bit gets transmitted, the TX goes low, which easily passes bot capacitor and diode, pulling reset low. After a few more bits, we have build a charge over the cap, and the circuit is not able to trigger reset any longer. The cap stay charged for "some time" and wont get discharged by anything going on on the TX line due to the diode. I did not use a super-duper cap, so it will loose it's charge over time and be ready to reset again after some seconds, fast enough to be ready for next time you wanna upload code.

 

Or you could have just bought one from me :D

The RST pin is the DTR via a capacitor. This USB interface was designed for the "Mini-Bots" but is sold seperately.

Super that this is a true "DTR RST" system......

.... Mine has 3.3Volt logic out ..... does this too.... or is there a magical switch (out of view) to select 3.3 or 5.0 ?

Please post a link here (or somewhere) to this module ;-)

The USB interface is  a 5V interface originally designed for the Mini-Bots.

To correct my previous statement, after checking the schematic, the RST pin is a direct connection to the DTR pin. This is because most Arduino boards have the series capacitor and pullup resistor built in.

I am not sure about the propeller chip but when using a 5V device to program a 3.3V ATmega controller, 4.7K resistors in series with the RX,TX and DTR pins will limit current enough that internal static protection diodes will prevent damage.

 

Gareth, once again your brilliance is appreciated and marveled at. Thank you! I have a unit that has been good for nothing but power that you've now made useful with this hack!

Nice, where did you get the module from?

The module came from this ebay link.......

..... however with delivery of over 6 weeks i would NOT recommend .....

.... just check to see if the module has the CP2102 chipset .... there are many out there.(search string :-"ttl uart CP2102")