Chapter 6 Parallel Input/Output Control

6.2.3Port Drive Strength Select

An output pin can be selected to have high output drive strength by setting the corresponding bit in the drive strength select register (PTxDSn). When high drive is selected, a pin is capable of sourcing and sinking greater current. Even though every I/O pin can be selected as high drive, ensure that the total current source and sink limits for the MCU are not exceeded. Drive strength selection is intended to affect the DC behavior of I/O pins. However, the AC behavior is also affected. High drive allows a pin to drive a greater load with the same switching speed as a low drive enabled pin into a smaller load. Because of this, the EMC emissions may be affected by enabling pins as high drive.

6.3Port Data Set, Clear and Toggle Data Registers

The port data set, clear, and toggle registers provide an alternate method for setting and clearing individual port I/O pins within a single port. Only port C and port E have data set, clear, and toggle registers. Figure 6-2should be contrasted with Figure 6-1to see the the effects of adding set/clear/toggle functionality to the port cell. SET_Enable, CLR_Enable, and Toggle_Enable are set when you write to the data set, clear, or toggle register, respectively. The bit pattern on the peripheral bus port is then used to perform the requested function on the port data register.

SET_Enable

PTxDDn

Output Enable

 

D

Q

 

 

 

CLR_Enable

 

 

 

 

 

 

 

 

TOGGLE_Enable

 

 

 

 

 

 

PTxDn

 

 

 

Module_Enable

D

Q

 

 

 

 

 

 

 

DATA

 

 

 

Output Data

 

 

 

 

 

 

 

 

 

1

 

 

 

Port Read

 

 

 

 

 

Data

 

 

 

 

 

 

 

0

Synchronizer

Input Data

 

BUSCLK

 

 

 

 

Figure 6-2. Parallel I/O Block Diagram Equipped with SET/CLR Functionality: Ports C & E

MCF51QE128 MCU Series Reference Manual, Rev. 3

Freescale Semiconductor

115

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Freescale Semiconductor MCF51QE128RM manual Port Data Set, Clear and Toggle Data Registers, Port Drive Strength Select