New Project wp34s micro usb flash cable revisited

[matthiaspaul]

(02-07-2015 11:16 AM)MarkHaysHarris777 Wrote:  Can anyone who has Harald's board installed tell me whether there is a crystal on the back side of the board?
[...]
The board I have has a crystal mounted on the back-side. Harald's pcb 2 the same?

Marcus has already answered this question, but I'd like to add, that, according to a photo of Harald's board (in another thread), he's using the 16-pin FTDI FT230X rather than the Prolific PL2303HX (which is for the most part pin-compatible with the 28-pin FTDI FT232RL).

Neither the PL2303HX nor the FT230X has a provision for an external crystal, whereas the FT232RL has (the PL2303HX has the pins, but they are documented as reserved and not connected). However, over the years I have never experienced problems omitting the crystal on the FT232RL when used in bus-powered or dynamically powered configurations (that is, when VBUS > 4V was available).

Quote:Another thing I'd like to know is whether he gets usb transmit distances of more than 50 cm (like across the room) or is that not possible?

USB 1.x/2.x is specified for up to 5m. Provided that proper USB cables (meeting the USB specs) are used, this will work with the FTDI as well as the Prolific chips, otherwise they weren't USB-compliant.

Actually, the TTL serial side between the USB chip and the ARM CPU is more critical than the USB side, since it is an unsymmetrical signal. While it is a very good idea to insert some series resistors into the lines for a number of reasons (to limit the current in case of external shortcuts or if someone would accidently connect two TX drivers, to help deal with the 3.0V/3.3V level transition, to reduce the current in case of an ESD event and generally to reduce ringing), I think, 1k is too much and may introduce problems if longer cables are used. I would recommend to not use more than 810R instead. Most probably, lower values would be sufficient as well, even if they would temporarily overload the CPU's output capabilities. In either case, the resistors should be located inside the calculator, not your external USB converter.

Quote:I guess at this point if I decide to place the IR transmitter in this thing, Harald's board is the way to go... otherwise, I am really liking Barry's idea of the tip-ring-gnd solution with a miniature gold phone jack (just for solid connection for flash|update).

Hm, the IR transmitter is just the IR LED and a resistor. It is controlled by a separate I/O port pin of the CPU, not by the TTL serial or USB interface. Instead of using the resistor on Harald's board, you could just as well solder the resistor onto the calculator's PCB or use a threaded resistor...

I too like Marcus' and Barry's 2.5mm 3-pin phone jack solution very much, but in order to play it safe, I would add some ESD protection diodes. Also, I would use a (3-pin compatible) 4-pin jack (female side: Tip = 3.0-3.3V RX, Ring 1 = 3.0V-3.3V TX, Ring 2 = 3.0-5.0V VCC, Shaft = GND) for an optional external power supply. (In case of 3-pin 3.5mm phone plugs, assembled cables with built-in USB converters are readily available on the market - I haven't seen them for 3-pin 2.5mm plugs or for 3.5mm or 2.5mm 4-pin plugs, though.)

Greetings,

Matthias