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Old 19th Jul 2017, 8:48 pm   #4
Karen O
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Join Date: Jul 2011
Location: Bridgnorth, Shropshire, UK.
Posts: 787
Default Re: MK14 (Microcomputer Kit 14) PIC14

Hello again Philoupat,

Your English is much better than my French so I'll reply in English

I have no plans to add a USB socket to my PIC14 but there is a good alternative. The FTDI company sell USB serial leads where the adapter chip is embedded in the USB connector. FTDI lead number TTL-232R-5V-AJ has a USB plug one end (to plug into a computer) and a 3.5mm stereo plug at the other. The 3.5mm plug carries 5V level serial data: the tip is data FROM the computer, the ring is data TO the computer (which you don't need in this case), and the sleeve is ground. This makes it very easy to add a serial link to a microcomputer.

You will need to connect the tip contact of a 3.5mm socket to pin 6. You don't need the transistor because the levels are already the right way round. As a precaution, I would add a schottky diode so that nothing is stressed if the computer is switched on when the PIC14 isn't (see diagram 1).

I notice that your PCB uses transistors to drive the digits. Today I would recommend using a 74AC240 IC instead. The problem with transistors is that they don't switch off quickly, and this causes 'ghosts' to appear on the display when you run some of the example programs in the Mk14 handbook.

I am sorry about this but there is a solution - put a 10nF capacitor over each base resistor as shown in diagram 2. These capacitors will speed up the transistor switch-off and cure the ghost problem. So as not to spoil the appearance of your PCB, you could solder the extra capacitors to the under side.

I hope all this helps.
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Last edited by Karen O; 19th Jul 2017 at 8:51 pm. Reason: Another thought occured to me
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