In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Sparks-R-Fun wrote:

Robert,

   It sounds like your approach should work.  If you like it, use it!  There are other methods to accomplish the same result.  Spending much time trying to identify the "best" method is probably not worth the effort.

  If my recollection is correct, I have created a similar signal-sharing scheme with a single quad-NAND-gate chip.  However, unless board space or shaving pennies off of the final cost is of great concern I would not say it is significantly better than what you have proposed.

   I understand your desire not to want to code another UART.  If you find it easier for your control signal to come from someplace other than the SX, a hardware solution seems like a good option.  However, if you can allow the SX to handle the control of the serial port, Bean's suggestion to use the built-in comparator is elegant in its simplicity!  Basically, he is suggesting using the comparator as a switch that either passes or blocks the serial data stream to the 680x controller.  

   To accomplish this just use resistors to feed a 2.5V reference signal to one input of the comparator.  The output will simply reflect whether the serial signal into the other input is above or below the reference signal.  It will therefore copy (or invert, depending on the configuration) the serial signal from the input to the output.  When you no longer want the serial signal to pass, simply disable the comparator and set the output pin to the appropriate state.  It should be very simple.

   Of course, you still have to know when to switch the serial signal!

 - Sparks

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