From: abe riazi (firstname.lastname@example.org)
Date: Fri Jul 21 2000 - 23:17:47 PDT
Included on Page 230 of "High-Speed Digital Design A Handbook of Black Magic" by H.W. Johnson and M. Graham, is Figure 6.7 intended to illustrate the "Right" and "Wrong" ways of applying parallel termination.
This idea is reflected by the attached gif picture. The two inserted marks !? imply it is theoretically valid but may not hold true in practice; as explained by the following paragraphs.
In theory, the optimum location for a parallel end terminator is beyond the chip shown by Figure A, as placing the terminator before the chip ( Figure B) is accompanied by undesirable capacitive loading effects of the stub separating the two components. Indeed, several important features of parallel termination (for instance its advantages over series termination which include allowing distributed load, undistorted waveform along full trace path and faster circuit performance)are optimally revealed when the terminator is located beyond the receiver as exemplified by Johnson and Graham. This theoretical superiority for placement of parallel terminators can be also easily verified via simulation. A useful reminder when simulating parallel or series termination; in the so called FF corner (i.e. Fast buffer and Fast environment) the largest value of the parallel and the smallest value of the series terminators (as dictated by each resistor's nominal value and tolerance) should be used. The opposite applies to the SS (Slow buffer, Slow environment ) simulation corner. Another noteworthy consideration for parallel termination is selection of one or two resistors schemes. Using one resistor often proves more economical in terms of cost, PCB space and power consumption; though, there are instances when use of two resistors (or Thevenin) is preferable. In parallel terminations involving one resistor the optimum termination voltage is often neither GND nor Vdd; instead an intermediate voltage Vtt = Vdd/2 which can require additional voltage source (regulator). Consequently, when only few signal lines need termination it can be advantageous to use two resistors (connected to GND and Vdd) rather than one demanding an extra regulator.
In practice, there are cases in which placement of the parallel termination past the receiver is not achievable. Placing the terminator before the chip requires only one time routing to the receiver, whereas positioning the resistor beyond the IC can impose routing under the chip twice (once from the driver to the receiver, and then from the receiver pin to termination). Therefore, for a receiver chip with a large number of pins (for example, BGA with more than 500 pins) the latter approach can prove impractical due to routing complexities.
In closing, Figure 6.7 of "High-Speed Digital Design A Handbook of Black Magic" indicates that the best location for a parallel terminator is beyond the last receiver IC (in a chain). This is valid in theory and verifiable by means of simulation. However, it has important practical limitations as placing the resistor past the chip requires routing twice, whereas positioning the terminator before the IC is achievable with only one time routing to the receiver pin.
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