Intel PXA255 manual Gpio Alternate Functions, General-Purpose I/O Block Diagram

System Integration Unit

When the processor enters sleep mode, the contents of the Power Manager Sleep State registers (PGSR0, PGSR1 and PGSR2) are loaded into the output data registers. If the particular pin is programmed as an output, then the value in the PGSR is driven onto the pin before entering sleep mode. When the processor exits sleep mode, these values remain driven until the GPIO pins are reprogrammed by writing to the GPDR, GPSR or GPCR, and setting the GPIO bit in the Power Manager Sleep Status register (PSSR) to indicate that the GPIO registers have been re-initialized after sleep mode. This is necessary since the GPIO logic loses power during sleep mode

Most GPIO pins can also serve an alternate function within the processor. Certain modes within the serial controllers and LCD controller require extra pins. These functions are hardwired into specific GPIO pins and their use is described in the following paragraphs. Even though a GPIO pin is used for an alternate function, you must still program the proper direction of that pin through the GPDR. Details on alternate functions are provided in Section 4.1.2. Figure 4-1shows a block diagram of a single GPIO pin.

Figure 4-1. General-Purpose I/O Block Diagram

		Pin Direction
		Register
		2
		Alternate Function
		Registers
	0	Pin Set and
	1	Clear Registers
GPIO Pin	2	Alternate Functions
	3	(Outputs)
	3	Alternate Functions
	2
	1	(Inputs)
	0	Edge Detect
	Edge
	Detect	Status Register
		Rising Edge Detect
		Enable Register
	Power Manager	Falling Edge Detect
		Enable Register
	Sleep Wake-up logic
		Pin-Level
		Register

4.1.2GPIO Alternate Functions

GPIO pins are capable of having as many as six alternate functions that can be set to enable additional functionality within the processor. If a GPIO is used for an alternate function, then it cannot be used as a GPIO at the same time. GPIO[0] is reserved because of its special use during sleep mode and is not available for alternate functions. GPIO[15:0] are used for walk-up from sleep mode. The walk-up functionality is described in Section 3.4.9.5, “Exiting Sleep Mode” on page 3- 18. Table 4-1shows each GPIO pin and its corresponding alternate functions.

4-2 Intel® PXA255 Processor Developer’s Manual