Forth Lesson 22

From OLPC

[edit] Using Forth Under Linux

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

[edit] On XO-1.75 and XO-4

On OLPC OS builds, Forth is in /runin/sdkit-arm and can be started from Terminal:

 $ su
 # cd /runin/sdkit-arm
 # RUNIN_PATH=/runin ./sdkit.sh
 ok 

Or download http://dev.laptop.org/~wmb/sdkit-arm.tgz . Unpack it into your home directory:

 $ tar xfz sdkit-arm.tgz

Then:

 $ cd sdkit-arm
 $ su
 # ./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"

[edit] GPIO Access

[edit] 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.

[edit] Camera Test

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

[edit] Accelerometer

[edit] Two-Wire Serial Interface (I2C/SMBUS) Hardware

[edit] 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.

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.

[edit] Recipes

[edit] Storage LED

Useful for checking that sdkit-arm is functioning.

 ok d# 10 gpio-set  \ turn on the LED
 ok d# 10 gpio-clr  \ turn off the LED
 ok d# 10 0 do d# 10 gpio-set d# 100 ms d# 10 gpio-clr d# 400 ms loop  \ blink the LED ten times

[edit] Thermal Sensor

Obtains the temperature from the CPU, until ticket #10954 is fixed.

 d4013200 unaligned-mmap constant thermal-base
 : cpu-temperature  ( -- celcius )
    0                     ( acc )
    d# 100 0  do          ( acc )  \ Accumulate 100 samples
       thermal-base @     ( acc reg )
       h# 3ff and         ( acc val )
       +                  ( acc' )
    loop                  ( acc )
    d# 52940 -  d# 196 /  ( convert to celcius )
 ;
 cpu-temperature .d

Thus endeth the lesson.