10. Parallel I/O
ÛZIATECH
58
Input Buffer
The input buffer is enabled during read operations to transfer the data from connector
J5 to the internal data bus. If the parallel port bit is configured as input, the data read is
the data driven by an external device.
The input buffer is an inverting device. This means that data read from the parallel port
as a logical 0 is a TTL high at connector J5, and data read from the parallel port as a
logical 1 is a TTL low at connector J5.
Debounce Control Logic
The debounce control logic is a feature of the 16C50A Digital I/O ASIC. This feature
eliminates the need for external logic or extensive software to remove unstable input
signals to the Digital I/O ASIC. The internal circuitry of the Digital I/O ASIC automatically
filters out glitches that can occur in received signals.
For example, if the Digital I/O ASIC is programmed for an 8 ms period, the incoming
signal must be stable for the entire 8 ms period, with no glitches, before it is recognized
by the Digital I/O ASIC.
The debounce control logic is controlled on the 16C50A by registers E0h, E1h, E2h, and
E3h in register bank 2. An 8 MHz clock is used by the 16C50A for a timing reference,
thus allowing the debounce circuit to be programmed for a debounce delay of 4 µs,
64 µs, 1 ms, or 8 ms.
Upon initialization of the debounce circuitry, be sure to delay at least the programmed
debounce time before reading any of the input ports or the external event signals. This
guarantees that the input data is valid prior to being used by the software.
Event Sense Detection Logic
The 16C50A Digital I/O ASIC contains event sense logic that allows detection of either
positive or negative events. The event input edge is controlled on a nibble basis by
software. The event bits are enabled on an individual basis.
Registers E0-E6h in Register Bank 1 are used to control and monitor the event sense
logic. The Event Sense interrupt can be assigned to IRQ15 by modifying the default
configuration of cuttable trace CT7. Be sure to see the topic "
)" in Appendix A for a table showing the proper configuration.
Note that modifying cuttable traces requires soldering ability. Incorrect modifications by
the user could void the product warranty. Contact Ziatech for custom configuration.