In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Zoot wrote:

Ummm.. where did the conversion number come from:

RawToCm 	CON 2257			' 1cm / 29.034us. see docu 
RawToMm 	CON 22572			' 1mm / 2.9034us. see docu 


Remember your ticks are 10us -- not 2us or .8us like on some of the Stamps. 

So.......hitting the docs:

[code]
Host Device Input Trigger Pulse tOUT 2 µs (min), 5 µs typical 
Echo Holdoff tHOLDOFF 750 µs ' quite a bit until pulse will start
Burst Frequency tBURST 200 µs @ 40 kHz 
Echo Return Pulse Minimum tIN-MIN 115 µs ' min pulse
Echo Return Pulse Maximum tIN-MAX 18.5 ms ' max pulse
 PING))) 
Sensor 
Delay before next measurement  200 µs ' could be good to know if they are being triggered too fast


RawToIn         CON     889                     ' 1 / 73.746 (with **)
RawToCm         CON     2257                    ' 1 / 29.034 (with **)

' At sea level sound travels through air at 1130 feet per second.  This
' equates to 1 inch in 73.746 uS, or 1 cm in 29.034 uS).

[/code]

So you want to divide in the form 1/29.034 where 1 = [b]1us[/b]. But you are counting [b]10 us[/b] increments. So you need to multiply the fractional constant by 10.

That means:

distance = raw ** ((1/2.9034)*$FFFF)
distance = raw ** ( 22571.812357925192 )
distance = raw ** 22572

But you need to divide by two, ideally before scaling, to minimize error:

distance = (raw+1)/2 ' cleaner rounding
distance = distance ** 8887

Picking a random sample of say 9000us (approx halfway through the Ping's useful distance), that should give something like 150 cm:

9000us = 900 ticks
( 900 ticks + 1 ) / 2 = 450 return trip
450 ** 22572 = 154.99198901350423
or 155 cm.

Sounds about right. ** is better if the fractional part is less than 1, i.e. multiplying by .7564 or something. */ is required if the fraction part is greater than 1, i.e. multiplying by 1.345. ** is more precise.

The rest looks OK. The anamalous data on the longer measurements is sure odd. Perhaps grasping at straws -- is the ping close to the floor/surface? What did you use for a test object? Are you just watching these values in the debugger? Without a "break" in a known location how do you know you are seeing the most recent full measurement, rather than a value in progress?

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