Browse Prior Art Database

Method to Select One of Two Clocks While Avoiding Narrow Pulses

IP.com Disclosure Number: IPCOM000099745D
Original Publication Date: 1990-Feb-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 3 page(s) / 73K

Publishing Venue

IBM

Related People

Gartmann, WS: AUTHOR [+5]

Abstract

The figures illustrate a method to select one of two asynchronous clocks using an asynchronous select signal while avoiding glitches.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 62% of the total text.

Method to Select One of Two Clocks While Avoiding Narrow Pulses

       The figures illustrate a method to select one of two
asynchronous clocks using an asynchronous select signal while
avoiding glitches.

      The logic schematic is provided in Fig. 1.  The corresponding
timing diagram is shown in Fig. 2.

      Two flip-flops and corresponding input gates form a state
machine whose state is indicated by -EN 1 and -EN 2. A state diagram
shown in Fig. 3 describes the state machine. CLK 1 indicates the
rising edge of the CLK 1 signal, and CLK 2 indicates the rising edge
of the CLK 2 signal.  If none of the conditions indicated by the
lines leaving each state are met, the state machine does not change
state.

      An output decoder uses the output of the state machine and the
two clocks to generate an output clock, as shown in Fig. 4.  There
are several items regarding this state machine worth noting.  First,
the state machine contains a DISABLED state which is entered each
time the selection changes from one clock to another.  Secondly, the
SELECT states are entered and exited only on the rising edge of the
clock which is selected by that state.  Finally, the state machine is
in the POSSIBLE POWERUP state only from power up until a positive
edge on CLK 1 or a positive edge on CLK 2.  In practice, the latches
generally have a reset input which is driven such that the state
machine is either in the SELECT 1 or SELECT 2 state upon power up.

     ...