Browse Prior Art Database

Method for Adding an External Real Time Clock to a System Having an Internal Real Time Clock

IP.com Disclosure Number: IPCOM000113115D
Original Publication Date: 1994-Jul-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 4 page(s) / 106K

Publishing Venue

IBM

Related People

Barakat, E: AUTHOR

Abstract

Disclosed is a method for adding an external Real Time Clock (RTC) to a system already having an integrated circuit with an internal RTC, in such a way that the external RTC can be used externally without causing bus contention during I/O reads from the RTC, and in such a way that the duplication of logic is not required, as the interface logic within the integrated circuit is used to interface with the external RTC.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Method for Adding an External Real Time Clock to a System Having
an Internal Real Time Clock

      Disclosed is a method for adding an external Real Time Clock
(RTC) to a system already having an integrated circuit with an
internal RTC, in such a way that the external RTC can be used
externally without causing bus contention during I/O reads from the
RTC, and in such a way that the duplication of logic is not required,
as the interface logic within the integrated circuit is used to
interface with the external RTC.

      As personal computer functions are increasingly integrated into
a single chip or into a few VLSI chips, the RTC is often viewed as a
logical function to integrate with other functions.  While this kind
of integration often yields a more reliable system because fewer
solder connections are required through the planar (system) board,
along with costs, a problem is created by integration if a function
which has been integrated is desired in a different form, external to
the integrated chip.  An example of this problem is found with the

RTC.  IBM ValuePoint* systems have a chip integrating the RTC with
other functions, such as DMA controllers, INT controllers, serial
port logic, and a timer/counter.  The problem arises when a different
RTC circuit is needed to provide more bytes of CMOS RAM and better
timekeeping accuracy.

      Fig. 1 is a schematic view of the integrated circuit 10 used as
described above in ValuePoint systems, with the logic needed to
interface with an RTC integrated within the circuit 10.  An external
crystal clock 11 supplies the RTC timing, and electrical power is
supplied to the RTC from the VDD voltage when external power is
present, or from a 3-volt battery 12 when external power is absent.
Integrated circuit 10 includes I/O ports 70H and 92H, decode logic
14, which generates port select signals for ports 70H and 92H, and
locking logic 16, which controls a Security Lock Latch, Boot Sequence
Lock Latch, and Password Protection, depending on the bits setup in
Port 92H.  The locking function is performed by blocking the RTCDS
and RTCWR signals to a internal RTC 18.  This blocking occurs when a
certain address or an address within a block of addresses within the
User CMOS RAM is detected.  Thus, while port 70H holds the address of
the loc...