uClinux on Blackfin BF533 Stamp - A DSP Linux Port (Part 2)

By Pramode C.E.

In Part 1 of this series, we had examined how to connect a BF533 Stamp board to our GNU/Linux machine and run a simple `hello, world' program. In this article, we shall look at writing simple device drivers to access the LED's and buttons on the board as well as manipulating the on-chip watchdog timer.

Compiling the uClinux kernel

You can download the uClinux kernel (with Blackfin support) from blackfin.uclinux.org/projects/uclinux533 - I am using the 2005R3 release.

Before you start compiling the kernel, make sure that you have set up the GNU toolchain for the Blackfin processor as described in Part 1; the Blackfin uClinux documentation project offers more information.

The kernel compilation process is documented in detail here - it's fairly standard procedure (make menuconfig; make). You can simply use the default settings most of the time; the only change I made was enabling loadable module support.

The kernel binary (in ELF format) will be present under the folder uClinux-dist/images; it will be a file named `linux'. Don't be surprised by the size of the file (around 5.6Mb in my case) - the file contains not only kernel code but also an elementary root file system which gets loaded onto a ramdisk when the board is powered up! It seems that this file system is built from the directory tree rooted at uClinux-dist/romfs/.

Downloading the kernel onto the board

Connect the Stamp board to the serial port of the PC and fire up `minicom'. As soon as you feed power to the board, a boot loader program called `uboot' starts running looking for keystrokes out of the serial port - if you hit `enter' in 5 seconds, `uboot' will suspend the booting process and display a prompt where you can enter some simple commands. The `print' command should show you several lines of the form `name=value'. We are interested in two such `names'; they are `serverip' and `ipaddr'. We shall assign values to them by typing:

set ipaddr 192.168.1.1
set serverip 192.168.1.2

The name `ipaddr' refers to the IP address assigned to the Ethernet controller on the stamp board and `serverip' refers to the IP address of the Ethernet card on the PC to which the board is connected. You should now check whether `uboot' is able to communicate with the PC via the Ethernet link by running:

ping 192.168.1.2

We have to set up a TFTP server on the PC and verify that it is working properly. As I am using Ubuntu, I had to `apt-get' three packages - tftp, tftpd and xinetd. (Note: the `apt-get' command is used to download and install packages on Debian GNU/Linux systems). The `xinetd' program should be instructed to start the TFTP daemon by creating a file called /etc/xinetd.d/tftp which contains entries of the form:

service tftp
{
	socket_type		= dgram
	protocol	= udp
	wait		= yes
	user		= root
	server 	= /usr/sbin/in.tftpd
	server_args = -s /boot
	wait		= yes
	disable	= no
}

The `server_args' line specifies that files to be download via TFTP should be placed under /boot.

Once this file is created, we should verify whether everything is working fine by starting `xinetd', copying the Blackfin uClinux kernel image to /boot and running tftp (on the PC):

$ cd /tmp
$ tftp localhost
tftp> get linux
Received 5676668 bytes in 0.9 seconds

Once we have verified that the Ethernet link between our GNU/Linux machine and the Stamp board is working OK (by pinging from uboot) and that the TFTP server on the PC also has been configured properly, we can reboot the board, hit `enter', get into the `uboot' prompt and type:

tftpboot 0x1000000 linux

This will download the newly created kernel image (called `linux') from /boot of our PC to the memory of the stamp board. Once this is over, we should type:

bootelf 0x1000000

and the board will boot with the downloaded kernel. Once we log onto the board, we should again configure the Ethernet controller with the proper ip address using the `ifconfig' command.

More info regarding `uboot' can be obtained from here.

Our first kernel module

Here is a simple `hello, world' loadable kernel module:

[Listing 1]

#include <linux/module.h>

int init_module()
{
	printk("Hello...\n");
	return 0;
}

void cleanup_module()
{
	printk("World...\n");
}

This module can be compiled with the help of the following `Makefile':

[Listing 2]

obj-m:=test.o
default:
	make -C /usr/local/src/uClinux-dist/linux-2.6.x/ M=`pwd`

We can `ftp' the resulting object file `test.ko' onto the stamp board and load it into the kernel by running `insmod ./test.ko'.

A minor problem

It seems that the file system on the board does not have the `mknod' command. It's very easy to build one ourselves:

[Listing 3]

#include <sys/types.h>
#include  <sys/stat.h>

main(int argc, char *argv[])
{
	int r;
	if(argc != 4) {
		printf("Usage: mknod file major minor");
		exit(1);
	}
	r=mknod(argv[1], S_IFCHR|0777, makedev(atoi(argv[2]), atoi(argv[3])));
	if(r < 0) {
		perror("mknod");
		exit(1);
	}
}

The code should be compiled like this:

bfin-uclinux-gcc -Wl,elf2flt mknod.c -o mknod

It can be ftp'd onto the board or can be made part of the file system by copying to uClinux-dist/romfs/bin and building the kernel once again.

Blinking LED's

The BF533 Stamp board comes with 3 LED's and 3 buttons attached to a few General Purpose I/O pins (or `programmable flags' PF0 to PF15 as per the Blackfin manual) - the LED's are on PF2, PF3 and PF4. The GPIO pins can be programmed via certain memory mapped registers. The pin direction (input or output) can be set by writing to a `direction register' at location 0xFFC00730 - if a bit of this register is `set', the corresponding pin acts as output and if it is clear, the pin acts as an input pin; for example, writing:

*((unsigned short*)0xFFC00730) = 0x1;

will result in PF0 being configured as output and all others as input.

The uClinux kernel for the Blackfin processor comes with macros using which we can access all these registers - the above expression can be rewritten as:

*pFIO_DIR = 0x1;

There are two other registers which we can use to set or clear the GPIO pins - writing a 1 to a bit of the FIO_FLAG_S register results in the corresponding GPIO pin going high and writing a 1 to a bit of the FIO_FLAG_C register results in the pin going low. All these registers can be accessed only from kernel space. Listing 4 is a simple character driver which sets or clears PF2 depending on a value it receives from user space. The file drivers/char/pflags.c in the Blackfin uClinux kernel source is a more complete implementation.

Programming the watchdog

A watchdog timer is a critical part of many applications which depend on the reliable operation of computer software - it's basically a timer which counts down to zero and resets the microprocessor when the count reaches zero - follow this link for more information.

The Blackfin CPU has a 32 bit watchdog timer. The registers associated with this timer are the watchdog count, status and control registers. The status register holds the current watchdog count value which gets decremented by one every clock cycle (the system clock is 100MHz on my board). Writing any value to this register when the watchdog is enabled results in the register being loaded with the value of the count register. Writing a value to the count register when the watchdog is disabled results in that value being copied to the status register. When the status register value becomes zero, the watchdog triggers an event which was previously selected by writing to a few bits of the control register (usually a system reset). The watchdog is enabled by writing any value other than 0xAD to bits D4 to D11 of the control register. Bits D1 and D2 of the control register decide the event to be triggered on timeout - setting the value to 00 chooses `system reset' as the event.

The working of the watchdog can be tested by writing a simple module whose init_module function contains the following lines:

*pWDOG_CNT = 500000000 // timeout in 5 seconds
*pWDOG_CTL = 0; // choose `reset' event and
 // enable watchdog.

The system will reboot five seconds after inserting the module!

Conclusion

We have seen how to do simple kernel programming on the BF533 Stamp board. Myself and Jesslyn (my student and author of the first part) would love to share with LG readers many more experiments using the Stamp board in later parts of this series!

Talkback: Discuss this article with The Answer Gang

---

As a student, I am constantly on the lookout for fun and exciting things to do with my GNU/Linux machine. As a teacher, I try to convey the joy of experimentation, exploration, and discovery to my students. You can read about my adventures with teaching and learning here.