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'[PIC]: multiplex UART'
2001\10\23@030317 by rasit gumus

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Hi all,

I need two uarts on a project , which i use 16F877 . Is there any way to multiplex the USART of the pic .



regards,

Rasit GUMUS

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2001\10\23@073529 by mike

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On Tue, 23 Oct 2001 09:49:41 -0700, you wrote:

>Hi all,
>
>I need two uarts on a project , which i use 16F877 . Is there any way to multiplex the USART of the pic .
Yes, as long as you only need to use one at a time - just use an
external mux (74HC157) controlled by a spare pin.
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2001\10\23@091632 by Thomas McGahee

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Rasit,

Someone already mentioned using a multiplexer IC.
You can also use OR gates to multiplex the USART TX. See
the attached GIF. You select the desired output by
applying a LOW to the associated OR gate control line.

This arrangement allows you to have both TX outputs OFF,
either one ON, or BOTH ON. When the control input to
the OR gate is HIGH, the output of the OR gate is forced
HIGH, which is the usual MARKING state.

I offer this as an alternative to using a multiplexer IC
in those cases where you might already have two OR gates
available, or where you might want to sometimes enable
BOTH TX outputs at once.

You may likewise use OR gates to multiplex two UART RX
signals in to the single PIC RX line. In this case you
need to ensure that the two OR gate outputs do not
fight one another. Connect the cathode of a diode to
each OR output. Connect the anodes together and to the
PIC RX line. Connect a 4.7k pullup from the PIC RX line
to +5.

You can share control lines to the corresponding
TX and RX multiplexers, so that selecting TX A
will also select RX A, for example.

Fr. Tom McGahee

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