Forth Lesson 22

Using Forth Under Linux

You can run Forth under Linux and use it to inspect I/O devices.

On XO-1.75

Get http://dev.laptop.org/~wmb/sdkit-arm.tgz . Unpack it into your home directory on an XO-1.75 .

 $ tar xfz sdkit-arm.tgz

Then:

 $ cd sdkit-arm
 $ sudo -s
 # ./sdkit.sh
 ok

sdkit comes up in hex mode, so you can enter and display numbers in hex without additional qualifiers. To enter a decimal number, precede it with "d# ", e.g. "d# 123"

GPIO Access

Command	Stack	Description	Example
gpio-pin@	( gpio# -- flag )	Flag is true if GPIO pin is high	d# 108 gpio-pin@ .
gpio-out?	( gpio# -- flag )	Flag is true if GPIO is an output	d# 108 gpio-out? .
gpio-set	( gpio# -- )	Drives GPIO high	d# 108 gpio-set
gpio-clr	( gpio# -- )	Drives GPIO low	d# 108 gpio-clr
gpio-rise@	( gpio# -- flag )	Flag is true if rising edge detected	d# 108 gpio-rise@ .
gpio-fall@	( gpio# -- flag )	Flag is true if falling edge detected	d# 108 gpio-fall@ .
gpio-edge@	( gpio# -- flag )	Flag is true if edge detected	d# 108 gpio-edge@ .
gpio-clr-edge	( gpio# -- )	Clears GPIO edge detector	d# 108 gpio-clr-edge
gpio-dir-out	( gpio# -- )	Sets GPIO direction to output	d# 108 gpio-dir-out
gpio-dir-in	( gpio# -- )	Sets GPIO direction to input	d# 108 gpio-dir-out
gpio-set-rer	( gpio# -- )	Enables rising edge detection	d# 108 gpio-set-rer
gpio-clr-rer	( gpio# -- )	Disables rising edge detection	d# 108 gpio-set-rer
gpio-set-fer	( gpio# -- )	Enables falling edge detection	d# 108 gpio-set-fer
gpio-clr-fer	( gpio# -- )	Disables falling edge detection	d# 108 gpio-set-fer

Multi-Function Pin Register Access

The Multi-Function Pin Registers (MFPRs) control the assignment of chip pins to internal functions, and also control characteristics of those pins, such as drive strength, pull up/down resistors, etc. Since each multi-function pins can be used as a GPIO, we use the GPIO number to designate which pin we are interested in.

Command	Stack	Description	Example
af@	( gpio# -- function )	Returns the MFPR setting for gpio#	d# 108 af@ .
af!	( function gpio# -- )	Sets the MFPR for gpio#	h# a0c0 d# 108 af!
dump-mprs	( -- )	Displays a table of all MFPR settings	dump-mfprs
gpio>mfpr	( gpio# -- address )	Returns the address of the MFPR register for gpio#	d# 108 gpio>mfpr .

Camera Test

The camera test turns on the camera chip and lets you access internal camera chip registers via its I2C bus interface.

Command	Stack	Description	Example
test-camera-i2c	( -- )	Turns on the camera and dumps its internal registers	test-camera-i2c
start-camera	( -- )	Sets up the camera clocks and pads, powers on and resets the camera sensor chip	start-camera
ov@	( reg# -- value )	Reads an internal camera register via I2C	5 ov@ .
ov!	( value reg# -- )	Writes an internal camera register via I2C	4 12 ov!
ov-dump	( -- )	Displays a bunch of camera registers	ov-dump

Accelerometer

Command	Stack	Description	Example
select /accelerometer	( -- )	Activate accelerometer device driver	select /accelerometer
acceleration@	( -- x y z )	Read acceleration values	acceleration@ .d .d .d acceleration@ .d .d .d cr many
unselect	( -- )	Deactivate the current device driver	unselect

Two-Wire Serial Interface (I2C/SMBUS) Hardware

Command	Stack	Description	Example
set-twsi-target	( slave channel -- )	Select a TWSI channel and slave address for subsequent access	h# 3a 6 set-twsi-target
twsi-b@	( reg -- byte )	Read a byte	h# 20 twsi-b@ .
twsi-b!	( byte reg -- byte )	Write a byte	h# 47 h# 20 twsi-b!
twsi-write	( byte_n-1 .. byte0 n -- )	Write n bytes from the stack	33 22 11 00 4 twsi-write
twsi-get	( reg#_n-1 .. reg#0 #reg-bytes #data-bytes -- data_n-1 .. data0 )	Send #reg-bytes register address bytes from the stack, then read #data-bytes onto the stack.	h# 23 h# 01 2 4 twsi-get . . . .

Accessing Arbitrary Devices

In addition to the "canned" access words for specific devices, you can manually access any device you wish, using the "mmap" command to assign a virtual address to the I/O device's physical address.

Command	Stack	Description	Example
mmap	( phys-addr size -- virt-addr )	Assigns a page-aligned virtual address to a physical device address range	d4050000 1000 mmap constant clock-unit-base
unaligned-mmap	( phys-addr -- virt-addr )	Assigns a possibly-unaligned virtual address to a physical device address	d4033800 unaligned-mmap constant twsi5-base

For mmap, the physical address and size must both be multiples of 0x1000 (the CPU page size), i.e. their low three hex digits must be 0.

For unaligned-mmap, the physical address need not be page-aligned. Unaligned-mmap calls mmap with a page-aligned address (mapping a single page) and then adds the offset into the virtual address.

You can add offsets to the virtual address to access registers within the range {virtual-address .. virtual-address+size-1}. For example:

 ok d4050000 1000 mmap  constant clock-unit
 ok clock-unit 24 + l@ .

You could use unaligned-mmap to get an address that points to an individual register within a register block, but it's usually better to map the base address of the register block and then apply small offsets to the virtual address to access individual registers.