DSP56321 datasheet(47/84 Pages) FREESCALE

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AC Electrical Characteristics

DSP56321 Technical Data, Rev. 11

Freescale Semiconductor

2-27

2.4.10 Considerations For GPIO Use

The following considerations can be helpful when GPIO is used.

2.4.10.1 GPIO as Output

•

The time from fetch of the instruction that changes the GPIO pin to the actual change is seven core clock

cycles, if the instruction is a one-cycle instruction and there are no pipeline stalls or any other pipeline

delays.

•

The maximum rise or fall time of a GPIO pin is 13 ns (TTL levels, assuming that the maximum of 50 pF

load limit is met).

2.4.10.2 GPIO as Input

GPIO inputs are not synchronized with the core clock. When only one GPIO bit is polled, this lack of

synchronization presents no problem, since the read value can be either the previous value or the new value of the

corresponding GPIO pin. However, there is the risk of reading an intermediate state if:

•

Two or more GPIO bits are treated as a coupled group (for example, four possible status states encoded in

two bits).

•

The read operation occurs during a simultaneous change of GPIO pins (for example, the change of 00 to 11

may happen through an intermediate state of 01 or 10).

Therefore, when GPIO bits are read, the recommended practice is to poll continuously until two consecutive read

operations have identical results.

Figure 2-26.

TIO Timer Event Input Restrictions

Figure 2-27.

Timer Interrupt Generation

TIO

481

480

TIO (Input)

First Interrupt Instruction Execution

Address

486

DSP56321 Datasheet(PDF) 47 Page - Freescale Semiconductor, Inc