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How to download and boot the Xinu Os from a BeagleBone Black
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Xinu is the elegant, minimalistic operating system.

Download xinu.boot on the BeagleBone

There are already instructions on the Xinu official page on how to download and boot Xinu from the BeagleBone Black(BBB). But I've had the following issues with it:

  • minicom wasn't able to download the file
  • after booting into Xinu, the watchdog would reset the BBB after ~60s

So here's what I did.

First, the instructions are only valid with the serial communication program minicom. There are other ways to download the boot image, but I'm only concerned with this one.

$ sudo apt-get install minicom

The serial cable should be connected to the BBB. This will (probably) show as /dev/ttyUSB0. Make sure you see it.

Enter the minicom setup with:

$ minicom -s

        | Filenames and paths      |
        | File transfer protocols  |
        | Serial port setup        |
        | Modem and dialing        |
        | Screen and keyboard      |
        | Save setup as dfl        |
        | Save setup as..          |
        | Exit                     |
        | Exit from Minicom        |

Select Serial port setup

| A -    Serial Device      : /dev/ttyUSB0                              |   
| B - Lockfile Location     : /var/lock                                 |   
| C -   Callin Program      :                                           |   
| D -  Callout Program      :                                           |   
| E -    Bps/Par/Bits       : 115200 8N1                                |   
| F - Hardware Flow Control : No                                        |   
| G - Software Flow Control : No                                        |   
|                                                                       |   
|    Change which setting?                                              |   

Press Shift+a (or A, for short) to change the Serial Device to /dev/ttyUSB0. Press F to disable the Hardware Flow Control. (it will automatically toggle to No). Press Esc to go to the previous menu and then Save setup as dfl. This will save the previous settings for the next time minicom is launched.

Lastly, the ymodem protocol used by minicom for file transfer uses /usr/bin/rb and /usr/bin/sb by default. If you don't have those installed, you'll get the failure executing protocol error when trying to upload the boot file. The easy fix is (thanks Andrew!):

$ sudo apt-get install lrzsz

Run minicom with noinit as admin:

$ sudo minicom -o

Next, follow steps 4-8 from the instructions mentioned above. If you somehow can't interrupt the boot process because you can't send any keys through minicom, check the "serial port setup" again.

Sometimes you'll get a timeout on pathname error from minicom. Just try again.

Do not follow step 9 yet. That is, do not issue the bootm command. If the watchdog is enabled on the BBB, after the Xinu boots, the watchdog timer will expire and the BBB will reset. Xinu does not have a "feed-the-dog" implementation as far as I can tell. The watchdog timeout is approximately 60 seconds.

Disable the BBB watchdog from uboot

You can check if the watchdog is set by reading the WDT_WSPR register, while in uboot (see also the Technical Reference Manual for AM335x). The WDT_WSPR has an offset of 48h from the base register, WDT1, which has the address 0x44e35000. So we need to check the 0x44e35048. From uboot (don't use the exact address, only 10h increments, otherwise you might get a reset):

U-Boot# md 0x44e35030 8

44e35030: 00001234 00000000 00000000 00000000    4...............
44e35040: 00000000 00000000 00004444 00000000    ........DD......

In this case, the value is 0x4444, which means the watchdog is enabled. To disable it,

U-Boot# mw 0x44e35048 0xaaaa; sleep 1; mw 0x44e35048 0x5555

There is only one trick though. The register WDT_WWPS.W_PEND_WSPR (offset 34h) must have a value different than zero when the second write (0x5555) is issued. Repeat the two mw steps above if the watchdog was not disabled.

Verify that the watchdog was disabled (i.e. the register was succesfully writen to with the value 0x5555)

U-Boot# md 0x44e35030 8

44e35030: 00001234 00000000 00000000 00000000    4...............
44e35040: 00000000 00000000 00005555 00000000    ........UUUU....

Now run the last step,

U-Boot# bootm

Modify Xinu to disable the Watchdog at startup

The step above gets tiresome quickly if you constantly need to boot Xinu and expect to do some work for more than a dozen seconds. For a more permanent solution you can modify the Xinu itself.

Add a new header file in the /include directory:

/* watchdog.h */

struct am335x_wdt {
        uint32  wdt_widr;        /* watchdog identification register    */
        uint32  res1[3];         /* reserved                            */
        uint32  wdt_wdsc;        /* watchdog system control register    */
        uint32  wdt_wdst;        /* watchdog status register            */
        uint32  wdt_wisr;        /* watchdog interrupt status register  */
        uint32  wdt_wier;        /* watchdog interrupt enable register  */
        uint32  res2[1];         /* reserved                            */
        uint32  wdt_wclr;        /* watchdog control register           */
        uint32  wdt_wcrr;        /* watchdog counter register           */
        uint32  wdt_wldr;        /* watchdog load register              */
        uint32  wdt_wtgr;        /* watchdog trigger register           */
        uint32  wdt_wwps;        /* watchdog write posting bits reg.    */
        uint32  res3[3];         /* reserved.                           */
        uint32  wdt_wdly;        /* watchdog delay configuration reg.   */
        uint32  wdt_wspr;        /* watchdog start/stop register        */
        uint32  res4[2];         /* reserved                            */
        uint32  wdt_wirqstatraw; /* watchdog raw interrupt status reg.  */
        uint32  wdt_wirqstat;    /* watchdog interrupt status register  */
        uint32  wdt_wirqenset;   /* watchdog interrupt enable set reg.  */
        uint32  wdt_wirqenclr;   /* Watchdog Interrupt Enable Clear Reg.*/

#define AM335X_WDT_ADDR 0x44E35000  /* Watchdog reg base address        */
#define W_PEND_WSPR     0x00000010  /* Write pending for reg WSPR       */

Specifying the complete watchdog register with the am335x_wdt structure might seem overkill, since I'm only needing access to two registers. Thus, two defines would have sufficed instead of the whole structure. But keeping everything consistent with the style and spirit of Xinu makes the extra work worthwile.

After adding the new header file, make sure to include it in the xinu.h, somewhere. I got something like this:

#include <clock.h>
#include <watchdog.h>
#include <mark.h>

Because all function declarations are to be found in /include/prototypes.h, this file should be modified as well:

/* in file wakeup.c */
extern	void	wakeup(void);

/* in file watchdog.c */
extern  void    wddisable(void);

/* in file write.c */
extern	syscall	write(did32, char *, uint32);

Add a new source file in the /system folder:

/* watchdog.c */

#include <xinu.h>

 * wddisable  -  disable the watchdog timer on BeagleBone Black
void wddisable() {
        volatile struct am335x_wdt *wdt_ptr =
          (volatile struct am335x_wdt *)AM335X_WDT_ADDR;

        /* execute the start/stop sequence for watchdog timers
           according to AM335X TRM instructions in */
        wdt_ptr->wdt_wspr = 0x0000AAAA;
        while (wdt_ptr->wdt_wwps & W_PEND_WSPR);
        wdt_ptr->wdt_wspr = 0x00005555;
        while (wdt_ptr->wdt_wwps & W_PEND_WSPR);

The only thing left to do is to actually call the wddisable defined above. The natural place is in some initialization function. The nulluser function in /system/initialize.c is a good place for it:

        /* Enable interrupts */


        /* Disable the watchdog */


        /* Create a process to execute function main() */

Run make and enjoy the Xinu.

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