SRF04 : connection & firing

Hi, referring to this thread : http://groups.google.com/group/comp.robotics.misc/browse_thread/thread/b78e53e29a37b8ee/95f866a65ff2ec6b?lnk=st&q=srf04&rnum=3#95f866a65ff2ec6b
I'm finally putting 6-8 SRF04 on my robot.
Now the question is : how many 68HC11 ports-pin do I need to fire them ?? The problem is that many ports-pin are already busy, then just some of them are free to use. ( this link show the free pins redboxed :
http://www.rasega.com/studio/robotica/port-libere.gif )
Thank You
Ric
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Whoop !! Nooobody can help me please ?? :-(
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The input pin to the SRF04 is a pin on the PIC (iirc) so the fan out from an HC11 pin to the PICs would be many. In short, one HC11 pin should be able to drive all 6 or more SRF04's. But! With all the SFR04's firing at once, you will have a hige spike in your power lead. Make sure to bypass each sonar with its own 100uF and .1uF cap across the power leads.
On the other hand, rhis is my best guess, realize I'm not speaking from experience. I've never fired more than one SRF04 at a time.
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Randy M. Dumse
www.newmicros.com
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Well ok, one pin shoul drive many sonars, but I can't use just one pin to receive the information back from sonars...this will lost information on direction ; hence I suppose it is possible to drive many sonars with just one pin, but I must also use as many pins as many angular sector I'd like to resolve, is it right ??
Does anybody ever tried using multplexing to grab the response from sonar one at time...hence using just 2 pins at all ?? (+ the pins to drive the multiplexer !!)
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I don't remember exactly the number of sonars you have. If I recall correctly it was 6 to 8. Let's say you had a maximum of 7. You might be able to use a 3-to-8 decoder to select the to generate your output strobe (pick 000 or 111 as strobe to all sonars) and a 8:1 multiplexer (with one unused channel) as a selector to a single input, to do the whole interface with 4 wires (3 outputs, and one input) and two external IC's. You could acutally use two CD4051's (or 74HC4052's if you can find them) and some resistors to accomplish the interface. You'd have to scan the sonars very quickly to see which outputs went low. But I can see this as being possible.
Short of using another micro to manage the ranging function, or converting the I/O to serial, such as using shift registers and the SPI (which might get you to 3 lines...maybe), I don't think you can get any simpler than 4 pins for 5,6, or 7 sonar rangers.
Don't forget your software will be quite complex and take a fair portion of the processors time managing this function.
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Randy M. Dumse

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