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MOTOROLA M68040 USER’S MANUAL 11-15
2.5 V
R = 50
50 T.L.
MC68040 OUTPUT
BUFFER
TYPICAL Z = 4–12 (LARGE)
0
TYPICAL Z = 25 (SMALL)
0
Figure 11-8. MC68040 Termination Network
If a designer uses alternative standard termination methods, such as RC termination
network (see Figure 11-9), Thévenin termination network (not illustrated), or no
termination method at all, which is not recommended, then the power dissipation of the
MC68040 will be significantly less than the large buffer terminated values. For termination
networks other than that illustrated in Figure 11-31, the designer must calculate the
component of power dissipated in the output buffer and add this value to the small buffer
unterminated value.
C = 300 pF
R = 50
50 T.L.
MC68040 OUTPUT
BUFFER
TYPICAL Z = 4–12 (LARGE)
0
TYPICAL Z = 25 (SMALL)
0
Figure 11-9. Typical Configuration for RC Termination Network
The following paragraphs describe how the large buffer terminated values were
calculated. The MC68040 termination network causes current flow through the output
buffer of the MC68040, regardless of whether the MC68040 is driving a logic one or a
logic zero. The following equation gives the large buffer termination network power
dissipation for a given pin:
I = (V ÷ (R + Zo)) + 5 mA
P=I
2
R
eff
Reff is the effective average output resistance, including typical pullup resistance, typical
pulldown resistance, and a duty cycle average of how often the pin is high, low, or three-
stated. Typical values for Zo are 6 for large buffer low output, 12 for large buffer high
output, and 25 for small buffer output. Using these values and duty cycle assumptions
based on sequential burst write cycles, Reff calculates to 7.7 for the MC68040 large
buffer mode and 25 for the small buffer mode.
Maximum termination current in the large buffer mode occurs for output:
Low: Itl = (2.5 V ÷ (50 + 6 )) + 5 mA = 49.6 mA
High: Ith = (2.75 V ÷ (50 + 12 )) + 5 mA = 50.8 mA
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