README.AVR32-port-muxing

AVR32 Port multiplexer configuration
====================================

On AVR32 chips, most external I/O pins are routed through a port
multiplexer. There are currently two kinds of port multiplexer
hardware around with different register interfaces:

  * PIO (AT32AP700x; this is also used on ARM AT91 chips)
  * GPIO (all other AVR32 chips)

The "PIO" variant supports multiplexing up to two peripherals per pin
in addition to GPIO (software control). Each pin has configurable
pull-up, glitch filter, interrupt and multi-drive capabilities.

The "GPIO" variant supports multiplexing up to four peripherals per
pin in addition to GPIO. Each pin has configurable
pull-up/pull-down/buskeeper, glitch filter, interrupt, open-drain and
schmitt-trigger capabilities, as well as configurable drive strength
and slew rate control.

Both controllers are configured using the same API, but the functions
may accept different values for some parameters depending on the
actual portmux implementation, and some parameters may be ignored by
one of the implementation (e.g. the "PIO" implementation will ignore
the drive strength flags since the hardware doesn't support
configurable drive strength.)

Selecting the portmux implementation
------------------------------------
Since u-boot is lacking a Kconfig-style configuration engine, the
portmux implementation must be selected manually by defining one of
the following symbols:

	CONFIG_PORTMUX_PIO
	CONFIG_PORTMUX_GPIO

depending on which implementation the chip in question uses.

Identifying pins
----------------
The portmux configuration functions described below identify the pins
to act on based on two parameters: A "port" (i.e. a block of pins
that somehow belong together) and a pin mask. Both are defined in an
implementation-specific manner.

The available ports are defined on the form

  #define PORTMUX_PORT_A	(something)

where "A" matches the identifier given in the chip's data sheet, and
"something" is whatever the portmux implementation needs to identify
the port (usually a memory address).

The pin mask is a bitmask where each '1' bit indicates a pin to apply
the current operation to. The width of the bitmask may vary from port
to port, but it is never wider than 32 bits (which is the width of
'unsigned long' on avr32).

Selecting functions
-------------------
Each pin can either be assigned to one of a predefined set of on-chip
peripherals, or it can be set up to be controlled by software. For the
former case, the portmux implementation defines an enum containing all
the possible peripheral functions that can be selected. For example,
the PIO implementation, which allows multiplexing two peripherals per
pin, defines it like this:

	enum portmux_function {
		PORTMUX_FUNC_A,
		PORTMUX_FUNC_B,
	};

To configure a set of pins to be connected to a given peripheral
function, the following function is used.

	void portmux_select_peripheral(void *port, unsigned long pin_mask,
			enum portmux_function func, unsigned long flags);

To configure a set of pins to be controlled by software (GPIO), the
following function is used. In this case, no "function" argument is
required since "GPIO" is a function in its own right.

	void portmux_select_gpio(void *port, unsigned int pin_mask,
			unsigned long flags);

Both of these functions take a "flags" parameter which may be used to
alter the default configuration of the pin. This is a bitmask of
various flags defined in an implementation-specific way, but the names
of the flags are the same on all implementations.

	PORTMUX_DIR_OUTPUT
	PORTMUX_DIR_INPUT

These mutually-exclusive flags configure the initial direction of the
pins. PORTMUX_DIR_OUTPUT means that the pins are driven by the CPU,
while PORTMUX_DIR_INPUT means that the pins are tristated by the CPU.
These flags are ignored by portmux_select_peripheral().

	PORTMUX_INIT_HIGH
	PORTMUX_INIT_LOW

These mutually-exclusive flags configure the initial state of the
pins: High (Vdd) or low (Vss). They are only effective when
portmux_select_gpio() is called with the PORTMUX_DIR_OUTPUT flag set.

	PORTMUX_PULL_UP
	PORTMUX_PULL_DOWN
	PORTMUX_BUSKEEPER

These mutually-exclusive flags are used to enable any on-chip CMOS
resistors connected to the pins. PORTMUX_PULL_UP causes the pins to be
pulled up to Vdd, PORTMUX_PULL_DOWN causes the pins to be pulled down
to Vss, and PORTMUX_BUSKEEPER will keep the pins in whatever state
they were left in by whatever was driving them last. If none of the
flags are specified, the pins are left floating if no one are driving
them; this is only recommended for always-output pins (e.g. extern
address and control lines driven by the CPU.)

Note that the "PIO" implementation will silently ignore the
PORTMUX_PULL_DOWN flag and interpret PORTMUX_BUSKEEPER as
PORTMUX_PULL_UP.

	PORTMUX_DRIVE_MIN
	PORTMUX_DRIVE_LOW
	PORTMUX_DRIVE_HIGH
	PORTMUX_DRIVE_MAX

These mutually-exclusive flags determine the drive strength of the
pins. PORTMUX_DRIVE_MIN will give low power-consumption, but may cause
corruption of high-speed signals. PORTMUX_DRIVE_MAX will give high
power-consumption, but may be necessary on pins toggling at very high
speeds. PORTMUX_DRIVE_LOW and PORTMUX_DRIVE_HIGH specify something in
between the other two.

Note that setting the drive strength too high may cause excessive
overshoot and EMI problems, which may in turn cause signal corruption.
Also note that the "PIO" implementation will silently ignore these
flags.

	PORTMUX_OPEN_DRAIN

This flag will configure the pins as "open drain", i.e. setting the
pin state to 0 will drive it low, while setting it to 1 will leave it
floating (or, in most cases, let it be pulled high by an internal or
external pull-up resistor.) In the data sheet for chips using the
"PIO" variant, this mode is called "multi-driver".

Enabling specific peripherals
-----------------------------
In addition to the above functions, each chip provides a set of
functions for setting up the port multiplexer to use a given
peripheral. The following are some of the functions available.

All the functions below take a "drive_strength" parameter, which must
be one of the PORTMUX_DRIVE_x flags specified above.  Any other
portmux flags will be silently filtered out.

To set up the External Bus Interface (EBI), call

	void portmux_enable_ebi(unsigned int bus_width,
			unsigned long flags, unsigned long drive_strength)

where "bus_width" must be either 16 or 32. "flags" can be any
combination of the following flags.

	PORTMUX_EBI_CS(x)	/* Enable chip select x */
	PORTMUX_EBI_NAND	/* Enable NAND flash interface */
	PORTMUX_EBI_CF(x)	/* Enable CompactFlash interface x */
	PORTMUX_EBI_NWAIT	/* Enable NWAIT signal */

To set up a USART, call

	void portmux_enable_usartX(unsigned long drive_strength);

where X is replaced by the USART instance to be configured.

To set up an ethernet MAC:

	void portmux_enable_macbX(unsigned long flags,
			unsigned long drive_strength);

where X is replaced by the MACB instance to be configured. "flags" can
be any combination of the following flags.

	PORTMUX_MACB_RMII	/* Just set up the RMII interface */
	PORTMUX_MACB_MII	/* Set up full MII interface */
	PORTMUX_MACB_SPEED	/* Enable the SPEED pin */

To set up the MMC controller:

	void portmux_enable_mmci(unsigned long slot, unsigned long flags
			unsigned long drive_strength);

where "slot" identifies which of the alternative SD card slots to
enable. "flags" can be any combination of the following flags:

	PORTMUX_MMCI_4BIT	/* Enable 4-bit SD card interface */
	PORTMUX_MMCI_8BIT	/* Enable 8-bit MMC+ interface */
	PORTMUX_MMCI_EXT_PULLUP	/* Board has external pull-ups */

To set up a SPI controller:

	void portmux_enable_spiX(unsigned long cs_mask,
			unsigned long drive_strength);

where X is replaced by the SPI instance to be configured. "cs_mask" is
a 4-bit bitmask specifying which of the four standard chip select
lines to set up as GPIOs.