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82C55A Port Hold Device Description
 
Background

The 82C55 Programmable Peripheral Interface uses internal port-hold devices to maintain the logic level of a port pin. This eliminates the need for pull-up resistors (see the NOTE below) to terminate un-used port pins. Port-A has both pull-up and pull-down devices, while Port-B and Port-C have only pull-up devices.

Specifications

The port-hold devices sink or source a current within the limits of the IPHH and IPHL specification. The port-hold high (IPHH) pull-up device will source current and the port-hold low (IPHL) pull-down device will sink current. The pull-up devices on all ports are turned on at power-up and during reset. Subsequent low logic levels on a Port-B and Port-C pin will turn off that pin's pull-up device until another reset is executed. Once the pull-up device is turned off the pin will not source current in excess of the input leakage specification (IIL). The pull-up and pull-down devices on Port-A will turn on in response to a logic-1 and logic-0, respectively. The Port-A pins will always source or sink the current specified by IPHH or IPHL.

The current necessary to overcome the port-hold devices is defined by the IPHHO and IPHLO specification. For example, if a pull-up device sources 300uA when active, it will require an external device to sink 350uA minimum to turn off that pull-up.

Note

The port-hold devices on the 82C55A are intended to maintain the logic level of un-used pins. Port pins loaded with more than 20pF of capacitance may not have their logic level guaranteed following a reset. When using the 82C55A in an NMOS system, the internal port-hold devices will not eliminate the need for pull-up resistors.

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