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Port A Bus Hold
The bus hold circuit on Port A of the Intersil 82C55 can hold a high or low state and is active as
long as the line is configured as an input. When the line changes direction from output to input, the
last value written to the line will determine whether the bus hold circuit pulls the line high or low. If
the last value written was a high voltage, the bus hold will try to keep the high value. If the last
value written was a low, the bus hold will try to keep the low value.
Special care is needed if open collector drivers are used. Open collector drivers actively drive a 0
to the line, but rely on a pull up resistor in order to drive a 1.
sink current = (VCC – 0.8) + 1.3 mA
R
ext_pullup
Where R
ext_pullup
is the value of any external pull-up resistance fitted.
If external pull-down resistors are used to hold the line low then the value of the pull down resistors
will need to be low enough to overcome the bus hold current. The hold current is strong (400
P
A
maximum) so the pull down resistors need to be <1.7 kohm.
If series resistors are used on the outputs of devices driving the data inputs then these resistors
values will also need to be reduced.
Port B and C Bus Hold
The bus hold circuit on ports B and C of the Intersil part can only hold a high state (‘1’) and is only
active from the time the line is configured as an input until the line recognizes a low value on the
input. Once the line is driven to a low value (by some external device), the bus hold for that line is
disabled and will not be enabled again until the port is reconfigured as an input. If the last value
driven to the line was high, the bus hold will try to keep the value high, so the line will remain high
when no driver is actively driving it.
If external pull down resistors are used to hold the line low then the value of the pull down resistors
will need to be low enough to overcome the bus hold current. The hold current is strong (400
P
A
maximum) so the pull down resistors need to be <1.7 kohm.
As this is a new feature of the 82C55, both new and existing users of the need to review
their input drive circuits to ensure they can source / sink sufficient current to overcome the ‘bus
hold current’ of the 82C55.
Summary of Contents for PCI230+
Page 2: ...PCI230 PCI260 ...
