XA User Guide 7-20 4/17/98
High Impedance Output
The final XA port output configuration is called high impedance mode. This mode simply turns
all output drivers on a port pin off. Thus, the pin will not source or sink current and may be used
effectively as an input-only pin with no internal drivers for an external device to overcome.
7.4.4 Reset State and Initialization
Upon chip reset, all of the port output configurations are set to quasi-bidirectional, and the port
latches are written with all ones. The quasi-bidirectional output type is a good default at power-
up or reset because it does not source a large amount of current if it is driven by an external
device, yet it does not allow the port pin to float. A floating input pin on a CMOS device can
cause excess current to flow in the pin’s input circuitry, and of course all port pins have input
circuits in addition to outputs.
7.4.5 Sharing of I/O Ports with On-Chip Peripherals
Since XA on-chip peripheral devices share device pins with port functions, some care must be
taken not to accidentally disable a desired pin function by inadvertently activating another
function on the same pin. A peripheral that has an output on a pin will use the I/O port output
configuration for that pin (quasi-bidirectional, open drain, push-pull, or high impedance).
The method of sharing multiple functions on a single pin involves a logic AND of all of the
functions on a pin. So, if a port latch contains a zero, it will drive that port pin low, and any
peripheral output function on that pin is overridden. Conversely, an on-chip peripheral outputting
a zero on a pin prevents the contents of the port latch from controlling the output level. It is
usually not an issue to avoid turning on an alternate peripheral function on a pin accidentally,
since most peripherals must be either explicitly turned on or activated by a write to one of their
SFRs. It is more likely that a user program could erroneously write a zero to a port latch bit
corresponding to a pin with a peripheral function that is being used and therefore disable that
function. The simple rule to follow is: never write a zero to a port bit that is associated with an
active on-chip peripheral, or that should only be used an input.
When an XA I/O port pin is used as an input for a peripheral function, it is sampled at the
oscillator rate divided by 2. For example, if an XA is running at a 20 MHz clock (giving a 50 ns
clock period), an external timer input would have to remain in the same state for at least 100 ns
in order to guarantee that it is sampled correctly. This gives a maximum frequency for such
inputs as the oscillator rate divided by 4. In this example, the maximum external timer input rate
would be 5 MHz.
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