FTDI breakout board

[fernando_g]

This idea evolved from the fact that I've projects which its external wall-wart power supply already provides regulated 5v.
To prevent connecting a 12v wall-wart into those projects, instead of the 2.5 or 2.1 mm circular receptacle, I use a mini USB instead.
Avoids damage.

In many of those projects I also have a download cable, which I always use my trusty AXE027.

But that got me thinking....If I am already using a mini USB connector o power the unit, why not use it as well for data?
Of course, I would have to use a breakout board on my project which in addition o providing the gnd and 5 v pins, would also have a FDTI chip to perform he USB to TTL conversion.

Is there anything unique in the Picaxe communication protocol which would prevent me from doing this?

 

[hippy]

That should work. It is effectively no different to putting the AXE027 inside the project case and pulling a 5V line out of it although an actual AXE027 does not easily allow for that.

The only thing you have to ensure is that your FTDI chip has the polarity inverted from its normal UART setting to use with the 'RS232' polarity the PICAXE expects and you may have to disable some of the other pins used for handshaking as well. You would also lose the ability to have software detect it as an AXE027 unless you programmed the chip to have the Rev-Ed PID/VID. Technically it is against the USB licencing to use a PID/VID for a device which isn't actually the device it is intended for.

There should be past discussion on the forum for doing as you suggest.

The biggest risk is if you use a high-current supply and then accidentally short +5V or something to 0V. We tend not to recommend USB supplies for that reason.

 

[Rick100]

Hello fernando_g,

I've used a Sparkfun FTDI breakout board for years. I don't think they sell the one I have anymore though. You may have to invert the TX and RX lines from their normal eeprom settings. I was using mine to program Arduinos and Picaxes and I can't remember what it's original polarity was. They require opposite polarities so I had to change them often. You use the FT_PROG utility from the FTDI website to reprogram the polarities. Here's a link to the page for FT_PROG.

http://www.ftdichip.com/Support/Utilities.htm#FT_PROG

I would make sure you get a real FTDI chip. The program may not work with fakes. I've been using the power banks meant for recharging cell phone batteries for power supplies. They're cheap and portable.

Good luck,
Rick

 

[fernando_g]

Thanks for the prompt replies.
Of course, I'm going to make sure that I purchase the real deal FTDI chip from reputable vendors.

Hippy, you bring a good point. Some sort of USB port power controller must be also included to prevent damage

 

[Goeytex]

Microchip, Atmel, TI, ST and others use USB 5V as a power source for their development boards. Modern USB chipsets are well protected against over current situations. For added protection these boards generally include a polyfuse. So I do not understand why it would not be recommended. I have been using USB power on my Picaxe dev boards for years without mishap.

A single USB 2.0 Port can by specification safely supply up to 500ma. I would therefore suggest using a 500ma polyfulse. This is what is included on many OEM dev boards.

You may also want to consider added ESD protection. LittleFuse is a leading supplier for this kind of stuff. Here's a pretty good document to look at.

http://www.littelfuse.com/~/media/electronics_technical/application_notes/usb/littelfuse_why_does_usb_2_0_need_circuit_protection_application_note.pdf

 

[hippy]

Modern USB chipsets are well protected against over current situations. For added protection these boards generally include a polyfuse. So I do not understand why it would not be recommended. I have been using USB power on my Picaxe dev boards for years without mishap.

Some USB power supplies are little more than a transformer and regulator and many will happily supply whatever current they are able to, regardless of fault condition in the target board.

If there is a short on the board, a mishap through a dropped bit of wire, incorrectly wired circuitry, or shorting something when probing a circuit, they can dump a couple of amps into the circuit which can fry things and even melt wires or tracking. These are very real risks with any development board.

Some people get away without ever experiencing a problem but everyone is prone to mistakes. I have to admit that my current limited bench supply has saved me from expensive damage a couple of times.

 

[Goeytex]

Some USB power supplies are little more than a transformer and regulator and many will happily supply whatever current they are able to, regardless of fault condition in the target board.

If there is a short on the board, a mishap through a dropped bit of wire, incorrectly wired circuitry, or shorting something when probing a circuit, they can dump a couple of amps into the circuit which can fry things and even melt wires or tracking. These are very real risks with any development board.

Some people get away without ever experiencing a problem but everyone is prone to mistakes. I have to admit that my current limited bench supply has saved me from expensive damage a couple of times.

Why then does Rev-Ed recommend/promote the use of AA batteries when they can dump from 5 to 10 amps given a short circuit condition? Also, I have seen little in the way of over current protection in Rev-ed example circuits or on Rev-Ed products. So your non-recommendation of USB power is therefore inconsistent and fails the logic test.

Any good design, regardless of the power source, should have some kind of over current protection. It's common sense.

IMO, Rev-Ed should be offering low cost USB powered Dev boards with the FTDI chip onboard ...along the lines of what Microchip and others have been offering for years. Oh, and please put a fuse on them.​

 

[hippy]

We would not recommend any batteries which can deliver a huge current over a sustained period. Our recommendation is against that.

Without over-current protection there is always some risk so we recommend minimising that risk wherever possible. In most cases using standard alkaline batteries the risk of serious problems under a fault condition should be minimised.

Adding over-current protection would add cost and can also be difficult to do when the actual current drawn is unpredictable and depends on what else is being connected.

 

[fernando_g]

I still believe that Hippy's comment is valid.
There are many cheap 5V USB wall-warts that are so low cost (read: poorly designed) that Rev-Ed's position should be a cautious one.

But since USB-powered gadgets are so ubiquitous, many experimenters will be eventually tempted to employ one.
Perhaps the Manuals should be amended to include such a powering option, with the caveat that a suitable polyfuse be put in place.

Thinking a bit further, perhaps RevEd one day will add an upgraded successor to the AXE027, whereas not only the proper TTL comm signals are available, but also the +5v power......protected with an internal polyfuse, of course.

Rick100: is the Sparkfun breakout board something like this one??
https://www.sparkfun.com/products/9716