NS486SXF Datasheet, PDF (25/44 Pages) National Semiconductor (TI) – NS486TMSXF Optimized 32-Bit 486-Class Controller with On-Chip Peripherals for Embedded Systems

3 0 Device Specifications (Continued)
3 3 AC SWITCHING SPECIFICATIONS
The following pages list some of the preliminary AC Specifi-
cations for the NS486SXF All parameters are listed in al-
phabetical order according to their Symbol
The Tables consist of the following
Parameter A short description of the specification be-
ing documented
Symbol
A quick reference between the timing dia-
gram and the Table entries
Formula
An equation which in addition to the Mini-
mum and Maximum Specifications can be
used to determine the actual timing provid-
ed at any operating frequency
Min
Minimum Specification when added to the
value produced by the formula
Max
Maximum Specification when added to the
value produced by the formula
How to calculate the actual specification at a given fre-
quency
In the formula column one will see many formulae which
contain the variable T The T represents one period (or
one T-state) of the CPU Clock So if the CPU is running
at 25 MHz T is equivalent to 40 ns similarly if the CPU is
running at 20 MHz T is equivalent to 50 ns
EXAMPLE Calculate the minimum guaranteed Column Ad-
dress Setup Time
At 25 MHz Formula a Min Spec e
(0 5T) a (b20 ns)
e
0 5 (40 ns) a (b20 ns) e
20 ns b 20 ns
e 0 ns
At 20 MHz Formula a Min Spec e
(0 5T) a (b20 ns)
e
0 5 (50 ns) a (b20 ns) e
25 ns b 20 ns
e 5 ns
As the frequency varies so will many of the specifications
One should always calculate the specification based on the
CPU’s operating frequency
25
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