Fellow PIC-Listers,

Based on the feedback I have received, it seems that a design like the following
 would make the most sense:

Break the SSU into two functional units -- the ranging unit and the
 communications unit.  Each would be based around a 16F84-10 PIC
 microcontroller.  A single 10 MHz oscillator chip will be used to drive both
 PICs.  Descriptions of each unit are as follows:

Ranging unit:
=============
10 MHz means a .4nS cycle time.  Using a prescaler value of 250 on Timer0 gives
 us a .1mS Timer0 interrupt period to use to keep track of our time-of-flight
 count.

Pin assignment will be as follows for this module:

Port A
------
  0 - Sonar Ping Out <-- Drives the out transducer at 40KHz through buffers.
  1 - Data Valid Out <-- Raised by PIC when data out is valid.  Lowered when
                          new ranging in progress.
  2 - Sonar Ping In  <-- Flag raised by analog ckt. when echo detected.
  3 - Range Mode In  <-- High = continuous, low = one-shot
  4 - Range Trigger  <-- Brought high momentarily to trigger one-shot ranging.

Port B
------
 0-7 - 8-bit Data Out

Communications Unit:
====================

Pin assignment will be as follows for this module:

Port A
------
  0 - Serial Data Out   <-- Serial Data Out to the world.
  1 - Range Mode Out    <-- Tells the Ranging Unit (RU) what mode to use.
  2 - Range Trigger Out <-- Triggers the RU when in one-shot mode.
  3 - Data Valid In     <-- Input from RU signals that data is valid.
  4 - Serial Data In    <-- Serial Data In from world. Using RA4 allows us
                             to use TMR0 rollover to detect start bit.
Port B
------
 0-7 - 8-bit Data In

Overview of Design:
===================

Having the Communications Unit (CU) and the Ranging Unit (RU) live on separate
 16F84-10 chips gives us three big advantages:

 1) We are free to use TMR0 for both serial communications timing
     and for time-of-flight timing.
 2) Different communication algorithms and protocols can be tried
     by simply substituting CU chips.
 3) Different range scaling factors or methods can be accomodated
     by simply substituting RU chips.

Additionally, as this is to be a collaborative effort, this allows different
 people to work on different pieces of the puzzle without worrying that their
 efforts will overlap or conflict.

Does anyone want to volunteer some programming expertise and development time?
 Anyone? Anyone?

 - Rick "Bueller?" Dickinson

+--------------------------------+------------------------------+
| Rick "The Notes Guy" Dickinson | Views expressed by the author|
| Lotus Notes & Domino Solutions | are not necessarily those of |
| (818)563-1061 rtd@notesguy.com | Enterprise ArchiTechs.  They |
|   http://www.eArchiTechs.com   | might not even be mine....   |
+--------------------------------+------------------------------+