The values on the pins that are configured as GPIO can be changed by using the following registers.

• GPyDAT Registers

  Each I/O port has one data register. Each bit in the data register corresponds to one GPIO pin. No matter how the pin is configured (GPIO or peripheral function), the corresponding bit in the data register reflects the current state of the pin after qualification. Writing to the GPyDAT register clears or sets the corresponding output latch and if the pin is enabled as a general-purpose output (GPIO output), the pin is also driven either low or high. If the pin is not configured as a GPIO output, then the value is latched but the pin is not driven. Only if the pin is later configured as a GPIO output is the latched value driven onto the pin.

  When using the GPyDAT register to change the level of an output pin, be cautious to not accidentally change the level of another pin. For example, to change the output latch level of GPIOA0 by writing to the GPADAT register bit 0 using a read-modify-write instruction, a problem can occur if another I/O port A signal changes level between the read and the write stage of the instruction. Following is an analysis of why this happens:

  The GPyDAT registers reflect the state of the pin, not the latch. This means the register reflects the actual pin value. However, there is a lag between when the register is written to when the new pin value is reflected back in the register. This can pose a problem when this register is used in subsequent program statements to alter the state of GPIO pins. An example is shown below where two program statements attempt to drive two different GPIO pins that are currently low to a high state.

  If Read-Modify-Write operations are used on the GPyDAT registers, because of the delay between the output and the input of the first instruction (I1), the second instruction (I2) reads the old value and writes the value back.

  GpioDataRegs.GPADAT.bit.GPIO1 = 1;  //I1 performs read-modify-write of GPADAT
  GpioDataRegs.GPADAT.bit.GPIO2 = 1;  //I2 also a read-modify-write of GPADAT
                                      //GPADAT gets the old value of GPIO1 due to the delay
  									
  

  The second instruction waits for the first to finish the write due to the write-followed-by-read protection on this peripheral frame. There is some lag, however, between the write of (I1) and the GPyDAT bit reflecting the new value (1) on the pin. During this lag, the second instruction reads the old value of GPIO1 (0) and writes the value back along with the new value of GPIO2 (1). Therefore, GPIO1 pin stays low.

  One answer is to put some NOPs between instructions. A better answer is to use the GPySET/GPyCLEAR/GPyTOGGLE registers instead of the GPyDAT registers. These registers always read back a 0 and writes of 0 have no effect. Only bits that need to be changed can be specified without disturbing any other bits that are currently in the process of changing.

• GPyDAT_R Registers

  The GPyDAT_R registers are read only registers that return the value written to the GPyDAT registers instead of pin status. Writes to these registers have no effect.

• GPySET Registers

  The set registers are used to drive specified GPIO pins high without disturbing other pins. Each I/O port has one set register and each bit corresponds to one GPIO pin. The set registers always read back 0. If the corresponding pin is configured as an output, then writing a 1 to that bit in the set register sets the output latch high and the corresponding pin is driven high. If the pin is not configured as a GPIO output, then the value is latched but the pin is not driven. Only if the pin is later configured as a GPIO output is the latched value driven onto the pin. Writing a 0 to any bit in the set registers has no effect.

• GPyCLEAR Registers

  The clear registers are used to drive specified GPIO pins low without disturbing other pins. Each I/O port has one clear register. The clear registers always read back 0. If the corresponding pin is configured as a general-purpose output, then writing a 1 to the corresponding bit in the clear register clears the output latch and the pin is driven low. If the pin is not configured as a GPIO output, then the value is latched but the pin is not driven. Only if the pin is later configured as a GPIO output is the latched value driven onto the pin. Writing a 0 to any bit in the clear registers has no effect.

• GPyTOGGLE Registers

  The toggle registers are used to drive specified GPIO pins to the opposite level without disturbing other pins. Each I/O port has one toggle register. The toggle registers always read back 0. If the corresponding pin is configured as an output, then writing a 1 to that bit in the toggle register flips the output latch and pulls the corresponding pin in the opposite direction. That is, if the output pin is driven low, then writing a 1 to the corresponding bit in the toggle register pulls the pin high. Likewise, if the output pin is high, then writing a 1 to the corresponding bit in the toggle register pulls the pin low. If the pin is not configured as a GPIO output, then the value is latched but the pin is not driven. Only if the pin is later configured as a GPIO output is the latched value driven onto the pin. Writing a 0 to any bit in the toggle registers has no effect.