Part 3- Fixing a failed 3D printer controller board

Fixing the Printrboard (finally!)

Now that I have an AVR programmer and the software to run it, it's time to hook it up to the failed Printrboard and see if I can fix it. However, there are a few things I wanted be cautious about:

Printer Settings:

Since the printer is hosed, I am not able to get its calibration setting as recommended in Step 4 of the Printrbot reflash guide. When the new firmware is installed, I will need to update the default steps per millimeter and also the printer build volume.

[Note: I actually bumped my volume settings up from 150.0 mm to 152.4mm).

Programming

Unlike prior versions of the Printrboard, the Rev D requires that its Boot jumper be installed when programming the firmware.

Power

A comment on Amazon's product page for the USBtinyISP specifically warns about the functionality of the JP3 jumper that is used to enable/disable +5 volts to the target (similar problems are mentioned for the SparkFun Pocket AVR Programmer where a 3.3v target board was destroyed, possibly caused by +5v leaking through the level shifter chip's output enable pins).

Adafruit's recommended use cases for removing the jumper are:

• The target is running at +5v, but needs more than the 100 mA of current that the USBtinyISP can supply and so the target needs to run off its own power supply.
• The target is not running at +5v (e.g., it has a 3.3v chip), so we will use the target's power supply to run the level shifters on the USBtinyISP.

Looking at the Printrboard schematics, I can see that the MCU is a 5 volt part, and also I found an article where someone was able to program their board with only the AVR programmer's power supply connected, so I will:

• Disconnect the +12V power cable at the Printrboard, but leave all other cables connected.
• Leave the JP3 jumper installed on the USBtinyISP .

Red Wire?

On the USBtinyISP, Pin 1 of the 6-pin ISP header is clearly marked and the cable that plugs onto it has its red wire at that position:

On the schematic, Pin 1 is the "MISO" signal and Pin 6 is "GND"

The Rev D Printrboard has a 6-pin SPI header labled "ICSP" (In Circuit Serial Programming), but there are no markings to indicate which is Pin 1. Looking at the schematics, the ISP header has Pin 1 connected to "GND" and Pin 6 connected to "MISO" (i.e., numbering is reversed compared to the AVR programmer!):

On the Printrboard artwork, the '1' marking on the ICSP JP4 connector *is* a ground pin;

therefore the "MISO" signal that runs on the red wire should go to the pin closest to the SD card; this would be consistent with how the RepRap wiki says the cable should be connected: 

"Connect the 6 pin programming cable to the Printrboard's ICSP header. 

Pin 1 (red wire) is closest to the SDcard slot."

[Note that the Printrboard artwork markings do not agree with how header pins are usually labeled with even-numbered pins in one row and odd-numbered pins in the other. Also, on the USBtinyISP, the location of the physical pins on the header does not match up with the graphical location on the schematic symbol. So "When in doubt, ohm it out".]

Which Bootloader to Install?

Lincomatic's blog has three different pre-compiled boot loaders to choose from:

• DFU : Atmel's officially supported bootloader
• CDC: Integrated with the Arduino IDE
• HID:  no drivers needed; plug and play

For this repair, I will use the CDC version since that integrates with my existing Arduino development environment.
[Note: once I have the CDC bootloader installed, I will not use Flip to install the printer firmware, but instead will use 'avrdude' via the 'avr109' protocol.]

What AVR Settings to Use?

The AVR chip operation is controlled by a set of internal fuses and lock bits described here (note that the fuses and lock bits use "negative" logic: un-programmed state is '1', programmed state is '0'). The settings are different for the factory DFU bootloader vs CDC bootloader:

          low / high / extended / lock

DFU:  "5e"  / "99"   / "f3" / "3f"

CDC:  "de" / "9b"   / "f0" / "3f"

From a AVR fuse calculator, I see that the differences are

• Clock divider:      DFU: divide by 8  / CDC: don't divide
• Boot flash size:     DFU: 4096          / CDC: 2048
• Brown out level:   DFU: 2.6 V         / CDC: 4.3 V

The difference that interests me is the "brown out level" since I think my firmware corruption could be caused by pulling the printer's power cord. Atmel's AVR documentation specifically mentions this as a method to protect the chip:

29.7.11 Preventing flash corruption

During periods of low VCC, the Flash program can be corrupted because the supply voltage is

too low for the CPU and the Flash to operate properly. These issues are the same as for board

level systems using the Flash, and the same design solutions should be applied.

A Flash program corruption can be caused by two situations when the voltage is too low. First, a regular write sequence to the Flash requires a minimum voltage to operate correctly. Secondly the CPU itself can execute instructions incorrectly, if the supply voltage for executing instructions is too low.

Flash corruption can easily be avoided by following these design recommendations (one is

sufficient):

1. If there is no need for a Boot Loader update in the system, program the Boot Loader bits to prevent any Boot Loader software updates.
2. Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done by enabling the internal Brown-out Detector (BOD) if the operating voltage matches the detection level. If not, an external low VCC reset protection circuit can be used. If a reset occurs while a write operation is in progress, the write operation will be completed provided that the power supply voltage is sufficient.
3. Keep the AVR core in Power-down sleep mode during periods of low VCC. This will prevent the CPU from attempting to decode and execute instructions, effectively protecting the SPMCSR Register and thus the Flash from unintentional writes.

Since I am using the CDC bootloader and using the recommended fuse settings, I hope I will no longer see problems due to cycling power.

As for the lock bits, a completely unlocked chip will have a setting of "3f" (only the lower 6 bits in the lock byte are used; the top two bits are always '0').  They can only be set to an unlocked state by erasing the chip. Erasure is done automatically when programming the chip from 'avrdude'.

Here We Go...

On the laptop where I have configured the USBtinyISP and avrdude:

• Download the printer firmware I will be installing. I got it from the Printrbot Simple Metal support page (file name "Printrboard_Auto-Level_Firmware_v2.hex", which I then renamed to "firmware.hex" for easier typing later on).
• Download the CDC bootloader zip file from Lincomatic's blog and extract "BootloaderCDC.hex".
• Disconnect the power cable from the Printrboard.
• Install the BOOT jumper onto the Printrboard (Rev D).
• Install the JP3 voltage jumper on the USBtinyISP.
• Connect the ribbon cable from the USBtinyISP to the 6-pin ICSP header on the Printrboard (red wire closest to the SD card slot).
• Connect the USBtinyISP to laptop with a USB A-B cable.
• Confirm that the USBtinyISP is connected (green LED on programmer).
• Confirm that the Printrboard is getting power from the USBtinyISP (green LED on target).

• Reset the Printrboard (press button on board).
• Open a command prompt on the laptop.
• 'cd' to the directory where I put the bootloader hex file.
• Run 'avrdude' to confirm that it can read the Printrboard (red LED flashes on programmer.

> avrdude -c usbtiny -p at90usb1286

• Check the state of the chip's fuses and lock bits:

> avrdude -c usbtiny -p at90usb1286 -U lfuse:r:-:h -U hfuse:r:-:h -U efuse:r:-:h -U lock:r:-:h

[Note: Fuses read as '0x5e' / '0x99' / '0xf3' ; lock bits are '0x24';
so the fuses are set to the factory DFU default, but the lock bits are not '0x3f'.
Instead a setting of' '0x24' means the the lock byte is configured as '00 10 01 00';
section 30.1 of the AVR chip spec shows that the bootloader is:

• Not allowed to write to the Boot Loader section.
• Not allowed to read from the Application section.
• Further programming and verification of flash and EEPROM is disabled.

• Re-program the chip's fuses for the CDC bootloader:

> avrdude -c usbtiny -p at90usb1286 -U lfuse:w:0xDE:m -U hfuse:w:0x9B:m -U efuse:w:0xF0:m

• Program the AVR with the new bootloader; this also erases the lock bits:

> avrdude -c usbtiny -p at90usb1286 -U flash:w:BootloaderCDC.hex:i

[Note: if avrdude complains of a verification error, ignore it: the USBtinyISP can write to a 128 KB chip, but can only read back 64 KB, which is why there is a mismatch at byte 0x1f000.]

• Reset the Printrboard (press button on board)
• Exit the command window
• Disconnect the USB cable from the laptop
• Disconnect the AVR ribbon cable from the Printrboard
• Make sure the boot jumper is still in place.
• Connect the power cable to the Printrboard.
• Connect the external power supply to the printer.
• Reset the Printrboard so that the board boots into the CDC bootloader
• Connect the laptop to the printer via USB.
• The printer should now install onto a new port using the same Teensy USB Serial Driver that I already print with (my printer uses "COM5" for printing, but the new bootloader port is "COM6".
• Open the command prompt window again and 'cd' to the directory that has the "firmware.hex" file. Use 'avrdude' to upload the firmware via the 'avr109' protocol over the 'COM6' port:

> avrdude -c avr109 -P COM6 -p at90usb1286 -U flash:w:firmware.hex:i 

• Exit the command window.
• Reset the Printrboard (press button on board).
• Remove the BOOT jumper from the Printrboard.
• Reset the Printrboard again so it starts up as a printer.
• Open Repetier, and see if it can connect to the printer.
• Use the manual control tab's g-code entry console to re-enter the original printer calibration settings (M92 steps and M211 build volume) and store them to EEPROM using the M500 command.
• Re-calibrate the Z offset (M212) and store it to EEPROM with M500.
• Print a test cube. 
• Be happy!