Motorola DSP56012 - Manual (Page 87)

Parallel Host Interface
Port B Configuration
MOTOROLA DSP56012 User’s Manual 4-7
4.2.2 Port B Data Direction Register (PBDDR)
For pins configured as GPIO by the PBC Register, the Port B Data Direction Register
(PBDDR) determines whether the pins are inputs (bit = 0) or outputs (bit = 1).
Note: The default setting after reset is input.
4.2.3 Port B Data (PBD) Register
The Port B Data (PBD) register provides access to the fifteen Port B pins as follows:
• If a pin is configured as a GPIO input and the processor reads the PBD
register, the processor sees the logic level on the pin. If the processor writes to
the PBD register, the data is latched there, but does not appear on the pin
because the buffer is in the high-impedance state.
• If a pin is configured as a GPIO output and the processor reads the PBD
register, the processor sees the contents of the PBD register rather than the
logic level on the pin, which allows the PBD register to be used as a general
purpose register. If the processor writes to the PBD register, the data is latched
there and appears on the pin during the following instruction cycle.
Note: If a pin is configured as a host pin, the Port B GPIO registers can help in
debugging HI operations. If the PBDDR bit for a given pin configured as an
input (i.e., 0), the PBD register shows the logic level on the pin, regardless of
whether the HI function is using the pin as an input or an output. If the
PBDDR is set (configured as an output) for a pin that is configured as a host
pin, when the processor reads the PBD register, it sees the contents of the PBD
register rather than the logic level on the pin—another case that allows the
PBD register to act as a general purpose register.
Note: The external host processor should be carefully synchronized to the DSP56012
to assure that the DSP and the external host properly read status bits
transmitted between them. There is more discussion of such port usage issues
in
Sections 4.4.4.7 HI Usage Considerations—DSP Side and 4.4.8.4 HI
Port Usage Considerations—Host Side .