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matb_uk
09-10-2007, 07:45 PM
Hi, Sorry if this has been answered already (I did have a scan through the forum).
I stumbled apon this site and the sweet looking Propeller chip and i've decided to take the plunch and order a couple. One question i have though is regarding the programming interface (Propplug). On the pdf doc for the prop it shows a schematic for an "Alternative Serial Port connection". Am i correct in assuming that this can be used to program the Prop ? (Possibly via a USB to Serial adapter which i already have) ? I only ask as i'm a bit of a skin flint and the cost of the programmer is twice as much as the chip itself.

Thanks,
Matt.

m.r.b.
09-10-2007, 08:02 PM
Basically ... Yes you can... Don't modify the circuit in any way... it works!!!

Its cheaper ... but the prop plug is 'off the shelf' and saves a little time... You have to work out the trade off between money VS time...


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B.T.W. as a side track....

You can also use a 'copy' of this circuit (minus the reset line wiring), on a couple of pins as a RS232 connection to a host PC for your application, using 'FullDuplexSerial' object. {I did this because my 'jellybean' MAX232 IC's in my stock are 5V devices, and are not 3.3V compatable!!!}
(When I have used the circuit as a comm port for my application, I have tried the circuit as is (-VE rail from handshaking pin), and also used the frequency synthesiser object, with a diode/capacitor charge pump to generate the comm -VE supply!! Both work.!!!)

Peter Jakacki
09-10-2007, 08:28 PM
The transistor circuit will work "plenty fine" if you connect the 4K7 resistor from the pnp collector to ground instead of DTR. Thus the circuit will work both as the prop programming port and as regular serial coms. The circuit as shown on the website uses the DTR in a non-standard manner.

As an engineer who does more than his fair share of work with serial coms & RS-232 both in hacked circuits and in commercial gear I still cannot understand why everyone seems to think you NEED the negative voltage other than to say that it is "compliant". It is really quite comical especially when you consider that the serial coms transmits at such a low bit-rate over such a short distance. Everything else we use is quite happy to send signals at much higher rates over the same distances with a small unipolar voltage. I have quite a few low-power low-part count circuits that couple the propeller's I/O directly to RS-232 signals (via resistors) and I have no trouble running the port at baud-rates higher than I could achieve with a MAX232 in the circuit.

Negative volts are redundant for normal operation of the PC's RS-232 receiver, it only needs to drop to a volt or less (no problem there). Also note that since the DTR is hooked up as a reset control you will get inadvertent resetting when closing the com port. The usual solution is to insert a switch in this path so that you can disable/enable DTR reset control.

*Peter*

matb_uk
09-10-2007, 08:34 PM
Thankyou for the replies. I'm ordering now.

hippy
09-10-2007, 08:40 PM
matb_uk said...
I only ask as i'm a bit of a skin flint and the cost of the programmer is twice as much as the chip itself.

I know exactly how you feel. The UK distributor has high mark-ups ( Propeller DIP 40 = $22, Proto Board = $38, Prop Plug = $47, including tax ) although postage and handling was fairly reasonable.

It feels a lot to pay for a USB to Serial interface, but at least you can be up and running almost as soon as you get your PropPlug. If you build your own interface and it doesn't work you are stuck in that frustrating area of not knowing if the interface isn't working or something else is wrong.

I gritted my teeth and bought a Prop Plug and I consider it to have been a worthwhile investment. You can always build a serial interface later and sell the Prop via eBay.

QuattroRS4
09-10-2007, 10:56 PM
I have used various methods - transistor circuit, prop plug and max3232 similar to the prop stick...
http://www.parallax.com/dl/docs/prod/prop/PropStick-v1.2.pdf

all have worked out fine.In the above schematic the 0.01uf cap is fairly important for the reset window - I have had .033uf working fairly ok but above that it is fairly inconsistent..

Peter has made a good point re: serial coms - you will probably notice that most usb to serial devices work between GND and +5v without issue..far outside the remit of a 'compliant' circuit..

Regards,
John Twomey

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'Necessity is the mother of invention'

Stan671
09-11-2007, 02:36 AM
matb_uk said...

i'm a bit of a skin flint and the cost of the programmer is twice as much as the chip itself
Yea, but you only need to buy one programmer and you can use it with lots of Prop chips. And the time you save by not building and testing your own can be used for playing with the Prop!

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Stan Dobrowski