![Digital Equipment Alpha 21164PC Скачать руководство пользователя страница 265](http://html.mh-extra.com/html/digital-equipment/alpha-21164pc/alpha-21164pc_hardware-reference-manual_2498508265.webp)
29 September 1997 – Subject To Change
Electrical Data
9–9
AC Characteristics
Figure 9–3 Input/Output Pin Timing
Because the speed and complexity of microprocessors has increased substantially
over the years, it is necessary to change the way they are tested. Traditional assump-
tions that all loads can be lumped into some accumulation of capacitance cannot be
employed any more. Rather, the model of a transmission line with discrete loads is a
much more realistic approach for current test technology.
Typically, printed circuit board (PCB) etch has a characteristic impedance of approx-
imately 75
Ω
. This may vary from 60
Ω
to 90
Ω
with tolerances. If the line is driven
in the electrical center, the load could be as low as 30
Ω
. Therefore, a characteristic
impedance range of 30
Ω
to 90
Ω
could be experienced.
The 21164PC output drivers are designed with typical printed circuit board applica-
tions in mind rather than trying to accommodate a 40-pF test load specification. As
such, it “launches” a voltage step into a characteristic impedance, ranging from 30
Ω
to 90
Ω
.
MK−1455−19
Input Timing
Tdsu
Tdh
50%
2.0 V
0.8 V
Output Timing
Tdd
50%
Vdd
2
Vdd
Vss
Vdd
Vss
Internal
Input
Signals
Output
Signals
CPU Clock
Internal
CPU Clock
Tcycle