Digital I/O, Programmable Peripheral Interface (PPI)
The programmable peripheral interface (PPI) is used for general purpose
digital I/O functions. This high performance TTL/CMOS compatible chip has
24 digital I/O lines divided into two groups of 12 lines each.
Group A:
Port A (8) lines and Port C upper (4) lines
Group B:
Port B (8) lines and Port C lower (4) lines
Port A, Port B and Port C are available at the 50 pin expansion connector.
You can use Ports A , B and C in one of these three operating modes:
Mode 0:
Basic I/O. Lets you use simple input output functions for a port. Data is
written to or read from the specified port
Mode 1:
Strobed Input/Output. Lets you transfer data I/O from port A in
conjunction with strobed or handshake signals.
Mode 2:
Strobed bi-directional input/output. Lets you communicate with an
external device through port A. Handshaking is similar to mode 1.
Available Port direction definitions:
Port A may be Inputs or Outputs
Port C lower bits may be Inputs or Outputs
Port C higher bits may be Inputs or Outputs
All these modes are discussed in detail in the 8255 datasheet from Intel.
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