TDI Web Log

Getting code onto the MidiMonster or Benito device.

Midi Monster Button Locations.	Benito 7g Switch Lcations
Benito 2010 Switch Locations	Benito Without Buttons

All of the code on the Benito and MidiMonster devices is open source and references an open source library called the Lightweight Usb For Avr (lufa). Getting the code compiled and onto the device requires a few other open source tools.

AVR-GCC

The most current and stable release of the toochain for the AVR has untill recently been maintained by Eric Wedddington and released as WinAvr (http://sourceforge.net/projects/winavr/) Winaver integrates nicely into atmels avr studio http://www.atmel.com/dyn/Products/tools_card.asp?tool_id=2725 and I reccomend that you get both if you are running windows. Each Winavr Release is closely followed by objective developments CrossPack for avr http://www.obdev.at/products/crosspack/index.html and a script for building the current toolchain on linux which is hosted by AvrFreaks at present there is also a debian package that was put out last month http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=90172

HWB? DFU?

Most Atmel “atmega” devices have a pin dedicated to determining whether or not the device goes into the bootloader depending on the devices flag settings. This pin is labled HWB (for hardware boot) on most of the datasheets. The usb avr family has this mode set up by default. When you hold the hwb pin low and reset the device it goes into the bootloader. On these devices the built in bootloader uses a usb device class called the Device Firmware Uploader (DFU). Atmel provides a tool called flip for programming DFU devices on windows and linux. There is an open source programmer called dfu-programmer http://dfu-programmer.sourceforge.net/

That other guys stuff.

Those of you who have worked with the teensy boards from paul stoffregon will notice some subtle differences. Paul wrote his own (closed source) bootloader rather than use the bootloader tha comes installed on the chips. Then he uses a hardware trick similar to the auto reset hack to make a single button manipulate both the reset and the hwb pins to put his boards into the bootloader. I didnt feel that either the two button arrangement or the builtin bootloaders were broken so I like most people working with these chips dont fix them.

Using the dfu-programmer

All of the programs in Dean Cameras Lightweight Usb for Avr have a “dfu” target. Once you have the target into the DFU mode you can simply

$make dfu

This will cause the dfu-programmer to erase the flash reprogramm it with a new hex file and restart the chip. You can also do this manually with the following commands

$dfu-programmer atmega32u2 erase
$dfu-programmer atmega32u2 flash mycode.hex
$dfu-programmer atmega32u2 start

This will cause the dfu-programmer to erase the flash reprogramm it with a new hex file and restart the chip.

I have a few devices that I need to interface to either the benito or another avr board that run at 3.3v. one is an ethernet interface, one is a bluetooth modem and then there are the x-bees.

One Way: Simple Voltage Divider.

When looking for interface examples for the xbee I found a site that started out with a circuit from maxstream and then once everything was working tried a resistor based voltage divider. This is how I got my first set of x-bees running via the ftdi chips.

One thing that this circuit didn’t do for me was to provide a decent pull-down for the reset. The 2 transistors on the right of this circuit are an attempt to do just that.

Another Way: Run everything at 3.3v

If you look at the datasheet for the atmega168 you will find the following diagram.

Looking at this you can see that 16mhz may or may not be in the operating range at 3.3v. But if you drop the crystal down to 8mhz you are good for sure. You may have to change a few things but it is doable.

The Ever Illusive Third: Buffers.

The idea of adding another layer at each interface between boards can have you asking for some Tzatziki and falafel to go with the PITA. There are buffers which are designed specifically for this. Many of the odd, many of them expensive. But there are also many families of buffer which are “tolerant” of a range of voltages outside of their supply. One of the buffers I checked out was the 74abt126 which is a tri state buffer. (I was looking at that because with the tristate buffers you can actually do a two way buffer by selecting the direction). I am not entirely certain that the 5v receive side is kosher with these.

The other buffers I am looking at with 5v tolerant i/o are the 74lvc125 and 74lvc126. These are 3.6v native parts with 5v “tolerant” io.

Since I ordered the wrong series of parts last group order I will have to wait a week to check this out.

See Also.

• Arduino Xbee Interface Circuit (AXIC)