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Support Forum » LCD Serial Backpack

October 12, 2009
by Ethanal
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I am thinking of buying the Serial LCD Backpack at this link. It will let me use more pins on my MCU by eliminating those six wires to the LCD and replacing them with one wire to RX. Is there any way to "multiplex" the RX pin so I can write to the LCD and the serial port for my computer separately?

October 12, 2009
by Ethanal
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P.S. Will this backpack enable me to write directly from the serial port of my computer to the lcd?

October 12, 2009
by rusirius
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I'll take a shot at answering your questions... ;)

You wouldn't really want to "multiplex" per se, but you could share the connection between PC and LCD..

You can use some FETs to "divert" the output to different devices... In other words, the output of the FETs would drive either the LCD or the serial, and then you can "enable" or "disable" the FETs... Group the Rx and Tx together to minimize extra pin use on the MCU... In fact, don't quote me cause I'm not sure this works except in my head, and I don't have any paper to try to draft it out real quick, but you may even be able to use P-channel and N-channel together in order to use only one additional PIN on the MCU... Of course if you want to make it a "manual" switch, then either two STSP swithes or one DTDP switch would do it...

As for the connection from the serial port, yes and no... Yes, it would technically, but keep in mind that it still needs TTL serial which is inverted from the normal PC serial...

October 14, 2009
by mikedoug
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Don't quote me either -- but you might could do something with the SPI (or similar busing technology) to talk to multiple devices on a common bus.



October 17, 2009
by pbfy0
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I wrote a software UART. here and here is the source.

I can't test, but no errors or warnings.

October 17, 2009
by mrobbins
(NerdKits Staff)

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As for the original goal of reducing the number of pins used for the LCD, I would highly recommend you take a look at this thread from our forums, where user wayward describes using a single 74LS174 IC (which is a ~$1 chip), in order to control the LCD with only 2 microcontroller pins! Haven't tried it personally, but he's posted photos and code, and I hope that helps / gives you ideas.

Additionally, as for the "LCD Serial Backpack" concept, don't forget that you could use a second microcontroller entirely for this purpose if you just write a little bit of code! Then, you could indeed just drive it directly from your PC serial port (or USB-Serial adapter). Best,


October 24, 2009
by pbfy0
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for second src, get that here.

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