ADSP-21368 SHARC Processor Hardware Reference 4-11
Digital Audio/Digital Peripheral Interfaces
When the pin enable is asserted, the pin output is logically equal to pin
input, and the pin is driven. When the pin enable is deasserted, the output
of the buffer amplifier becomes high impedance. In this situation, an
external device may drive a level onto the line, and the pin is used as an
input to the ADSP-21367/8/9 and ADSP-2137x processors.
While the pin is at high impedance and another device is driving a logic
level onto the external pin, this logic level value is sent to the SRUs as the
pin interface output. Even though the signal is an input to the SHARC
processor, it is an output from the pin interface (as a three-terminal
device) and may be patched to the signal inputs of peripherals within the
SRUs. Pin output is equal to pin input when the pin enable is asserted,
but pin output is equal to the external (input) signal when the pin enable
is deasserted.
L
If a DAI/DPI pin is not being used, the pin enable (for example
DAI_PBxx_I) for its pin buffer should be connected to LOW and its
associated bit in the DAI_PIN_PULLUP register should be set (= 1) to
enable a 22.5 kΩ pull-up resistor for that pin.
Pin Buffers as Signal Output Pins
In a typical embedded system, most pins are designated as either inputs or
outputs when the circuit is designed, even though they may have the abil-
ity to be used in either direction. Each of the DAI/DPI pins can be used as
either an input or an output. Although the direction of a DAI/DPI pin is
set simply by writing to a memory-mapped register, most often the pin’s
direction is dictated by the designated use of that pin. For example, if a
DAI pin is hard wired to only the input of another interconnected circuit,
it would not make sense for the corresponding pin buffer to be configured
as an input. Input pins are commonly tied to logic high or logic low to set
the input to a fixed value. Similarly, setting the direction of a DAI pin at
system startup by tying the pin buffer enable to a fixed value (either logic
high or logic low) is often the simplest and cleanest way to configure the
SRUs.
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