Datasheet

7–181
Recommendations for Designing More-Reliable Systems
Bus Control
The simplest way to avoid floating inputs in a bus system is to ensure that the bus always is either active or inactive for a limited
time when the voltage buildup does not exceed the maximum V
IL
specification (0.8 V for TTL-compatible input). At this
voltage, the corresponding I
CC
value is too low and the device operates without any problem or concern (see Figures 2 and 4).
To avoid damaging components, the designer must know the maximum time the bus can float. First, assuming that the
maximum leakage current is I
OZ
= 50 µA and the total capacitance (I/O and line capacitance) is C = 20 pF, the change in voltage
with respect to time on an inactive line that exceeds the 0.8-V level can be calculated as shown in equation 1.
DVńDt +
I
OZ
C
+
50 mA
20 pF
+ 2.5 Vńms
The permissible floating time for the bus in this example should be reduced to 320 ns maximum, which ensures that the bus
does not exceed the 0.8-V level specified. The time constant does not change when multiple components are involved because
their leakage currents and capacitances are summed.
The advantage of this method is that it requires no additional cost for adding special components. Unfortunately, this method
does not always apply because buses are not always active.
Pullup or Pulldown Resistors
When buses are disabled for more than the maximum allowable time, other ways should be used to prevent components from
being damaged or overheated. A pullup or a pulldown resistor to V
CC
or GND, respectively, should be used to keep the bus
in a defined state. The size of the resistor plays an important role and, if its resistance is not chosen properly, a problem may
occur. Usually, a 1-k to 10-k resistor is recommended. The maximum input transition time must not be violated when
selecting pullup or pulldown resistors (see Figure 3). Otherwise, components may oscillate, or device reliability may be
affected.
=
V
CC
R
V(t)
C
T
V
CC
R
BUS
Figure 8. Inactive-Bus Model With a Defined Level
Assume that an active-low bus goes to the high-impedance state as modeled in Figure 8. C
T
represents the device plus the
bus-line capacitance and R is a pullup resistor to V
CC
. The value of the required resistor can be calculated as shown in
equation 2.
V(t) + V
CC
–[e
–tńRC
T
(V
CC
–V
i
)]
Where:
V(t) = 2 V, minimum voltage at time t
V
i
= 0.5 V, initial voltage
V
CC
=5 V
C
T
= total capacitance
R = pullup resistor
t = maximum input rise time as specified in the data sheets (see Figure 3).
(1)
(2)