Use of the RCtime Command - Capacitor Discharge

' Program RCTIME_1.BAS
' Charges external capacitor to near +5.0 VDC.  On executing the RCtime
' command, the IO pin goes to a high impedance and capacitor discharges
' toward ground with a time constant RC.  When the voltage decays below
' the logic threshold (V_thresh) such that it is seen as a zero by the 
' BasicX the time is passed back to the calling function in T_fall.
' Note that T_fall = RC * ln (5.0/V_thresh) where V_thresh is nominally
' 1.4.  Thus, knowing T_fall and C, the value of R may be determined.
' One problem with this technique is that the value of V_thresh may vary
' from one device to another.  This may be remedied by using a calibration;
' that is, measuring T_fall with a known resistor.
' Another technique is to charge the capacitor and let it discharge toward
' ground; T_fall and to then discharge the capacitor and let it charge 
' toward 5V; T_rise and then sum the two values.
' This is treated in RCTIME_2.BAS.
' Copyright, Peter H. Anderson, Baltimore, Sept, 99

Public Const P24 as byte = 24	' RCTime pin

Sub Main()

   Dim T_rise as Single
   Dim T_fall as Single
   Dim T_tot as Single

   Call OpenSerialPort(2, 9600)		' For debugging

      Call PutPin(P24, bxOutputHigh)	' exert a one
      Call Sleep(1.0)			' give it time to charge

      Call RCtime(P24, 1, T_fall)
      Call PutS(T_fall)
      Call NewLine()
End Sub