Freescale Semiconductor, Inc.

Freescale Semiconductor, Inc.

GENERAL PURPOSE I/O CONFIGURATION

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Figure 5-6 Instructions to Write/Read Parallel Data with Port B

Figure 5-7 details the process of programming Port B as GPIO. Normally, it is not good programming practice to activate a peripheral before programming it. However, reset activates the Port B general-purpose I/O as all inputs; the alternative is to configure Port B as an HI, which may not be desirable. In this case, it is probably better to insure that Port B is initially configured for general-purpose I/O, and then configure the data direction and data registers. It may be better in some situations to program the data direction or the data registers first to prevent two devices from driving one signal. The order of steps 1, 2, and 3 in Figure 5-7 is optional and can be changed as needed.

5.2.2Port B General Purpose I/O Timing

General purpose data written to Port B is synchronized to the central processing unit (CPU) but delayed by one instruction cycle. For example, the instruction

MOVE DATA15,X:PORTB DATA24,Y:EXTERN

1.writes 15 bits of data to the Port B register, but the output pins do not change until the following instruction cycle

2.writes 24 bits of data to the external Y memory, which appears on Port A during T2 and T3 of the current instruction

As a result, if it is desirable to synchronize Port A and Port B outputs, two instructions must be used:

The NOP can be replaced by any instruction that allows parallel moves. Inserting one or more “MOVE DATA15,X:PORTB DATA24,Y:EXTERN” instructions between the first and

5 - 8 PORT B MOTOROLA

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Freescale Semiconductor, Inc.

Freescale Semiconductor, Inc.

GENERAL PURPOSE I/O CONFIGURATION

second instruction effectively produces an external 39-bit write each instruction cycle with only one instruction cycle lost in setup time:

One application of this technique is to create an extended address for Port A by concatenating the Port A address bits (instead of data bits) to the Port B general-purpose output bits. The Port B general-purpose I/O register would then work as a base address register, allowing the address space to be extended from 64K words (16 bits) to two billion words (16 bits +15 bits = 31 bits).

STEP 1. ACITIVATE PORT B FOR GENERAL - PURPOSE I/O:

*Reserved; write as zero.

Figure 5-7 I/O Port B Configuration

MOTOROLA PORT B 5 - 9

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