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Commodore PET Projects - petSD+

petSD+ - Firmware

Last Firmware Update : 01/12/2016 Get it here Revision History here


The petSD+ NODISKEMU firmware is based on sd2iec, created by Ingo Korb (et al.), the most common use for this firmware is in the emulation of a Commodore 1541 disk drive for the Commodore 64 computer, but Nils Eilers has added support for the IEEE-488 disk drives used with the PET/CBM range.

sd2iec is open source, released under the GNU GPL license and is available from the Git repository here. Unfortunately, there isn't a manual for sd2iec, although some information is available from the README file in the archive and some more "user friendly" information has been posted to the sd2iec page on the C64 wiki.

Quoting from the wiki, some features relevant to the PET are :

  • D64/D71/D80/D81/D82 image file support
  • PRG/P00 etc. support
  • Limited REL support
  • Supports FAT long file names
  • Supports FAT12/FAT16/FAT32 formatted SD and SDHC cards
  • Supports most typical IEC functions (LOAD, SAVE, OPEN, GET, PUT, CLOSE, ...)
  • Real Time Clock (RTC) support

This page describes how to load petSD+ firmware onto a blank MCU, i.e., one that does not already contain a compatible copy of the sd2iec boot-loader. Upgrading an operational petSD+ with updated firmware is a trivial task - jump to the end of this page for details.


Implementation for petSD+

Based on discussion with Ingo Korb, for petSD+, Nils has created a fork of the sd2iec firmware called NODISKEMU with its own repository on Github. As of  June 2015, this branch of sd2iec is still under development, it includes the enhanced IEEE-488 routines and D80 / D82 support from Nils' petSDduo firmware ("emily") but at this point, does not use the LCD display (work on this is in progress).

A comparison of the features of the petSD+ NODISKEMU firmware with those of alternative PET SD card solutions can be found on my firmware comparison page (currently a "work in progress").


Loading NODISKEMU to petSD+

This is made up of two distinct operations :

  • Program the AVR with a boot-loader
  • Place a copy of the NODISKEMU on SD card


The sd2iec Boot-loader

petSD+ uses the boot-loader that is part of the "standard" sd2iec project. Each time that the device is powered up, the boot-loader scans the root directory of the SD card, looking for a copy of the binary file for the application.

The files in the root directory are read, if the file size matches the expected length of a firmware binary and the file signature (which includes version, checksum, and firmware type) is valid, it can potentially be used to upgrade the firmware in the AVR. If the firmware file is compatible, then, provided that the version number is 0, or is greater than that currently installed in the AVR, the flash is reprogrammed with the updated firmware.

If an updated firmware file is not present, the system attempts to run the code currently in the flash. 


Step-by-Step Instructions - courtesy of Nils Eilers


Download the boot-loader from its official download page
  Download the latest "newboot-binaries" file Version 0.4.1 or greater is required
      e.g., Unzip the file and extract the required hex file
      for petSD+, the file is newboot-0.4.1-petSD+-m1284p.hex (or later)


Program the AVR flash with the boot-loader

As described on my Tools page, the hardware needed to program the AVR depends on whether you need to edit and recompile the code, or just need to download pre-compiled code as a binary file.

For petSD+, we just need to download the hex file, so we can use AVRDUDE.


  Download the appropriate version for your system from :
      For Windows  
          Download the latest file with "mingw32" in its name e.g.,  . . . . . /avrdude/
          Unzip the file to where you want to run it from  
          avrdude.exe and avrdude.conf will be created (The files are also compatible with 64 bit Windows)
      For Unix like OSs, use the appropriate command


      Debian GNU/Linux
        OS X with MacPorts
        OS X with homebrew
sudo apt-get install avrdude
sudo pkg install avrdude
sudo port install avrdude
brew install avrdude --with-usb


The documentation for AVRDUDE (PDF) is available from :
    and also available on-line (HTML) at :       . . . . . . /avrdude/user-manual/avrdude.html
AVRDUDE is a command line utility and uses a number of command line switches, with a large number of options, to control its execution. The number of options may appear quite daunting, but are relatively easy to use and well described in the documentation, the specific requirements for petSD+ are detailed below.

However, should you  prefer to use a graphical user interface GUI), a Java program called Burn-O-Mat is available for the OSs mentioned above at :

Burn-O-Mat has an on-line fuse calculator available on its webpage, but reading / writing of the MCU requires that the application is installed on your computer. Since Burn-O-Mat uses Java, the Java SE Runtime Environment (JRE), the appropriate version of JRE for your operating system must be installed and the configuration file edited to include the ATMega1284P used in petSD+.

    JRE can be downloaded from :
Command Line Execution

Since the whole point of using AVRDUDE in this instance is to program a device that is itself intended for use with a computer without a GUI, then using a command line tool should not be too much of a hardship.

Windows users : since working with the command line is quite unusual these days, a reminder of how Copy & Paste works in Windows command windows might be useful - click to pop up in a new window


AVRDUDE Parameters  
The minimum parameters that AVRDUDE needs are :  
  ("cable", "-c")
  (-P <port>)

Examples :

       -c stk200   -P lpt1
         -c avrispmkii
         -c usbasp
   (-p  partno)

for petSD+, this should be

the type of programmer being used
which computer port the device is attached to
legacy Atmel STK200 parallel port programmer
AVR ISP mkII (-P not needed for USB)
USBASP (-P not needed for USB)
the type of controller (MCU) connected
-p m1284p   OR   -p m1284
Testing communication with the Programmer

Before trying to program the device, it is a good idea to check that you can successfully communicate with the programmer on its own by sending it simple commands, for example, to set the speed of how fast the programmer will communicate with the MCU, using a USB connected Atmel AVR ISP mkII for example, you would enter :


avrdude -B 50 -c avrispmkii -p m1284p  (-B specifies the bit clock period in s)

This value is very slow, but should work under all circumstances. If we try to write to the programmer at too high a rate (with the -B value too low), the write will fail.

In the example below under Windows, with no programmer connected, avrdude prints out a failure message, in this case, reporting that it could not find the programmer on the USB port

Note : A similar error will be reported if the USB drivers for the programmer have not been installed. If you have not installed the drivers for the USB programmer, see my petSD+ Tools page for details of how to do this.

With the programmer available, but no MCU connected, avrdude reported the warnings and errors shown below :

The "target doesn't answer" error is hardly surprising - as the MCU was not connected, however, the "cannot set sck period. please check for usbasp firmware update" warning is worth mentioning.

If you "Google" that warning message, you will find that it is a common problem with the cheap Chinese USBASP programmers such as mine. It seems that many, if not all, of those cheap programmers come with an older version of the firmware which does not support setting the SCK frequency. For programming petSD+, the warning is not a barrier to doing the job, it is just an annoyance. When I programmed the first set of petSD+ MCUs, I just ignored it, but I quickly got fed up seeing the warning and upgraded the programmer's firmware as described here.

The screen dumps from this point onwards were taken after the USBASP firmware has been updated.

With the MCU connected, you should get output such as :

The program output shown above, confirms that AVRDUDE can communicate with the MCU and has read the device signature and current fuse settings. In this case, the device signature is correct for an ATmega1284P (1e9705) and the fuse configuration corresponds to the default values for the device. If the MCU has previously been used, the fuse settings may be different, but there should be no error messages reported.
We can now set the fuses to the required configuration for petSD+ using :

        avrdude -c usbasp -p m1284p -U lfuse:w:0xF7:m -U hfuse:w:0xd2:m -U efuse:w:0xFC:m
AVRDUDE will generate a lot of output (if you are interested, click on the image to see the full listing) as the fuse data is read, written and verified. You can ignore most of this, but the output should end with the last two lines shown, i.e.,

        avrdude: safemode: Fuses OK (E:FC, H:D2, L:F7)
        avrdude done.  Thank you.

If setting the fuses was successful, we can proceed to loading the boot-loader program. If there were any errors reported when setting the fuses, then these must be fixed first!

Windows users : please ensure that AVRDUDE is able to find the boot-loader's hex-file. The easiest way of doing this is to  place the file downloaded previously, e.g., newboot-0.4.1-petSD+-m1284p.hex, into the same directory as the AVRDUDE executable (.exe) file.

Once the fuses are set, we're able to program at full speed with -B 1:

        avrdude -c usbasp -B 1 -p m1284p -U flash:w:newboot-0.4.1-petsd+-m1284p.hex 
If no errors are reported, then the hardest part is done!

All that remains is to copy the application binary file to the root directory of a FAT formatted SD card.

Application Program (NODISKEMU) Installation / Upgrade

Boot-loader operation

Once the petSD+ MCU contains a compatible copy of the sd2iec boot-loader, loading and upgrade of the NODISKEMU application firmware does not require the use of any programming hardware or software. Each time that petSD+ is powered on, the boot-loader looks for an updated firmware file on the SD card as described in the sd2iec boot-loader readme file :-

"[The] boot loader searches for files with the correct length in the root directory of the card. It does not care about file names at all, instead it checks a tag at the end of the file to determine if it should flash a file or not.

The boot loader will only flash a file that is tagged for the correct hardware and that has either

 a) a version number that is greater than the one currently in memory, or

 b) a special version number that designates the file as a "development version".

In case b) the boot loader will avoid flashing the same file over and over again, but a development version file with a different checksum will get flashed, as well as any non-development version if the program version currently in the chip. FAT16 and FAT32 are always supported, FAT12 only if enabled. MMC, SD and SDHC cards with a supported file system should all work.

The boot loader turns on the red LED while it is running. If it cannot find a valid application in the chip, it will flash the red LED for two seconds and try to find a valid file once more. During card accesses the green LED is on, during the actual flash operation the green LED flickers rapidly."

Eventually, the release version of the petSD+ firmware will be available for download from this page, however, at this point, Nils is still actively developing the firmware and a "release" version does not yet exist.

NODISKEMU's sources are available from Once a day, these sources are compiled for all supported hardware variants if there are any changes. Those binaries are available for download from the nightlies repository on Nils' site.

Install / Upgrade NODISKEMU

Delete any existing firmware files from the root-directory of your SD-card  
Download the required binary from :
Either UnZip the download to the root directory of your SD card, or  
          Copy the file from its current locationto root directory of your SD card  
Note : the archive contains binaries for both petSD and petSD+ Use the file named petSD+.bin
Remove the SD card from your PC  
      Windows users : be sure to : Safely remove hardware
Insert the card into your petSD+ and switch it on  
The bootloader will program the firmware that is suitable for your device (Even if both petSD and petSD+ are present)
The binaries can remain onto the card, as describer above, the boot-loader won't program a binary over and over again. Since the files aren't required any longer, they can also be safely deleted.


References :-

sd2iec home page

sd2iec boot-loader readme file from the boot-loader archive file

C64 Wiki - sd2iec firmware page



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