Confused at the function "pulseIn()"

hello everyone, working on rc signal mixed system. , found useful library function: pulsein()
i interested @ function's completed, found source code @ arduino-1.0.5/hardware/arduino/cores/arduino/wiring_pulse.c
i confused solution returned.

code: [select]


return clockcyclestomicroseconds(width * 21 + 16);


and annotate says:
       // loop has been determined
   // 20 clock cycles long , have 16 clocks between edge
   // , start of loop. there error introduced by
   // interrupt handlers.

then why width multi 21?? why not 22 or multi 20 , plus constant? how guys work out it?

in case, code in assembly make immune compiler changes , count number of cycles.

in lieu of coding in assembly, use avr-objdump create assembly listing , count cycles.  use python script wrap avr-objdump (named "ds.py" dump sketch)...

code: [select]


import fnmatch
import os
import subprocess
import string

pathok = false
pathlist = os.environ['path'].split( os.pathsep )
for path in pathlist:
  testme = os.path.join(path,'avr-objdump.exe')
  if os.path.exists(testme):
    pathok = true
    break

if not( pathok ):
  arduinoroot = 'c:\\arduino\\arduino-1.0.5\\'
  pathlist.append( arduinoroot + 'hardware\\tools\\avr\\utils\\bin' )
  pathlist.append( arduinoroot + 'hardware\\tools\\avr\\bin' )
  pathlist.append( arduinoroot + 'hardware\\tools\\avr\\etc' )
  pathnew = os.pathsep.join( pathlist )
  os.environ['path'] = pathnew

tempdir = os.environ.get('temp')
tempfile = os.path.join(tempdir,'junk.cpp')

matches = []
for root, dirnames, filenames in os.walk(tempdir):
  filename in fnmatch.filter(filenames, '*.elf'):
    matches.append(os.path.join(root, filename))

matches.sort( key=os.path.getmtime )

if len(matches) > 0:
  f = open(tempfile, 'w')
  subprocess.call(['avr-objdump.exe','-s','-z',matches[len(matches)-1]],stdout=f)
  f.close()
  os.system( 'start ' + tempfile )



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