FAST-20 Active Negation Current

Paul D. Aloisi 603-429-8687 aloisi at hawk.uicc.com
Tue Jan 24 05:30:22 PST 1995


Tak, Ron,

John Lohmeyer has covered the specification, there is no minimum 
Active negation current specified in SPI. The pull up from the
terminators is a maximum of 24 mA each, the minimum is about 20 mA.
The signal step size require to reach the threshold in one step is
65 mA. The 2.0 volt limit for 22 mA minimum was to insure that the
second step did not drive the signal too high. 

There is a second issue with the receiver thresholds. Bill Ham's 
tests had shown the receiver thresholds had to be moved higher for 
FAST-20 to work with the specifications for active negation drivers 
|from SPI. A driver that can sink 65 mA can not drive the line below 
1 Volt on the first step. Bill Ham's test showed the center point 
for the thresholds should be 1.6 Volts. 

The threshold move was voted down in the FAST-20 meetings. A half way
step was finally approved after several meetings. This was still a 
problem. If the thresholds could not be moved up, then the only other
thing that could be done is to lower the maximum drive. This would
reduce the starting point for the first step, which would allow a 
typical 65 mA driver sink to reach the 1.0 Volt minium threshold in
one step.

The driver maximum voltage of 2.0 - 3.24 Volts max 7 mA restricts the
high drive.

The soruce/sink terminator required for the active negation drivers
can hold the maximum voltage and mimimize overshoot. 

The main issue addressed was making FAST-20 work with the receiver 
thresholds voted on by the committee. It was a signal integrity
issue to balance the drive and voltages on the cable. SPI allows
too much overshoot and does not require adequate drive for FAST-20
single ended to work with 8 (25 pF) devices connected to a cable.

Paul D. Aloisi
Unitrode
 




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