Amplicon Liveline PCI230+, Инструкция по эксплуатации

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4.7.3 Intersil 82C55 Differences from Oki 82C55
In order to meet RoHS compliance, the OKI 82C55 chip originally fitted on non-RoHS compliant
revisions of this board has been replaced with an Intersil 82C55 chip. The Intersil 82C55 has an
added feature to the I/O buffer called Bus Hold which can hold either a 0 or 1 depending on the
circuitry attached.
The bus hold feature affects the behavior of the device when its data lines are configured as
inputs. When a digital I/O line is configured as input, its value is normally defined by the driver
driving that line. When there is no driver, then the value can be defined using pull resistors (a pull
up will define a ‘1’ value, and a pull down resistor will define a ‘0’ value). The boards with the
Intersil part have additionally a feature called a “bus hold”. This element will remember the last
value that line was driven to and will try to hold that value by driving the line weakly (similar to a
pull resistor).
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.
If users need to ensure that any of the I/O pins are low at power up, (e.g. when the I/O pin will later
be defined as an output that needs to be low at power up) they need to provide the means
externally to ensure that the line is pulled down low with enough resistance to overcome the bus
hold current.
Because the bus hold currents are quite strong, existing pull downs may not be strong enough to
overcome the bus hold feature.
The bus hold feature may have unwanted effects on your implementation, if:
x
At power up the I/O lines need to be in a low state
x Pull down resistors are used to hold the I/O line in a known state.
x Input driving circuits have insufficient current source / sink capability.
In all other cases, the application should NOT be affected by this specification change.
The bus hold implementation on Port A is different to that used on ports B and C.