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Browse Prior Art Database

Dynamic Signal Clamp Circuit

IP.com Disclosure Number: IPCOM000108661D
Original Publication Date: 1992-Jun-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 3 page(s) / 74K

Publishing Venue

IBM

Related People

Murata, H: AUTHOR [+2]

Abstract

Disclosed is a new signal clamp circuit for an off-chip driver of TTLs. This invention reduces spurious switching noise due to reflection.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 73% of the total text.

Dynamic Signal Clamp Circuit

       Disclosed is a new signal clamp circuit for an off-chip
driver of TTLs.  This invention reduces spurious switching noise due
to reflection.

      Practical transmission lines and cables used for TTL
interconnections have various impedances between 30 ohms and 150
ohms.  For the majority of interconnections in digital systems, the
impedances of the transmission lines are less than the input and
output impedances of the TTLs.  Owing to the mismatch of the
impedances, the reflected waves travel back and forth on the
transmission line.  Practically this is observed as an overshoot and
undershoot at the specific point on the transmission line.  A
transmission line with a driver and a terminating resistor is shown
in Fig. 1.  A signal ringing caused by reflections is observed at
point B in Fig 1.  Overshoots and undershoots are periodically
observed because of the multiple reflections (Fig. 2).

      Fig. 3 shows a circuit for implementing the invention. Two
Schottky diodes are added to the output of the bus driver.  One is
connected to two voltage sources through a switch; the other is
directly connected to ground.  When the level of the output goes
under 0V, Diode 2 becomes active (SWITCH_ON) to clamp the undershoot.
It should be noted that a standard TTL driver has only Diode 2, and,
therefore, only the undershoot is clamped.

      Diode 1 enables the circuit to clamp the overshoot as well.
The switch to Diode 1...