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Programmable On-Chip Capacitance Control for Real Time Clock

IP.com Disclosure Number: IPCOM000118567D
Original Publication Date: 1997-Mar-01
Included in the Prior Art Database: 2005-Apr-01
Document File: 2 page(s) / 44K

Publishing Venue

IBM

Related People

Kimura, S: AUTHOR [+2]

Abstract

Disclosed is a method to tune the capacitance which is required for the on-chip oscillator to keep accurate time. The tuning of the on-chip capacitance can be performed through the Capacitance Control Register which is involved in the internal circuit.

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Programmable On-Chip Capacitance Control for Real Time Clock

      Disclosed is a method to tune the capacitance which is required
for the on-chip oscillator to keep accurate time.  The tuning of the
on-chip capacitance can be performed through the Capacitance Control
Register which is involved in the internal circuit.

Generally, the frequency of the oscillator can be expressed as
follows:
         F = 1/ ( 2 * pi * SQRT(L1*C1*(CL+C0)/(C1+CL+C0)) )
        CL = (Ct1 * Ct2) / (Ct1 + Ct2)
   Ct1 = Cx1 + C_uncontrol1
   Ct2 = Cx2 + C_uncontrol2

      L1, C0 and C1 are referred to as the electrical equivalent of
the mechanical parameter of a crystal.  C_uncontrol1 and C_uncontrol2
are floating capacitances depending on wiring, lead pins, and so on.
Cx1 and Cx2 are the sum total of the on-chip capacitance controlled
through the Capacitance Control Register at both X1 and X2 sides.  In
terms of this expression, it is proved that the fine tuning of Cx1
and Cx2 is extremely important for accurate time keeping.

      The Figure shows the overall circuit structure including the
on-chip oscillator, the on-chip capacitors, the Capacitance Control
Register and the Real Time Clock.  The Capacitance Control Register
fetches the control data given from the outside and individually
controls PFETa, PFETb, PFETc and PFETd.  Then, it determines the
condition for attachment or detachment of the on-chip capacitors, Ca,
Cb, Cc and Cd,  which means the decisi...