Computing stuff tied to the physical world

Assembling the JeeSMD, part 2

In AVR, Hardware on Jul 9, 2010 at 00:01

Yesterday’s post was about assembling all the SMD components of the JeeSMD kit.

The last step is to program a sketch into the ATmega. This isn’t as straightforwards as with a JeeNode, because there’s no on-board FTDI or USB serial port hookup.

It’s fairly easy to create an FTDI connection, but even if you do, you’ll still need an ISP programmer to install a boot loader (see this recent post for some background).

So let’s hook up an ISP programmer first:

Dsc 1787

I’m using a somewhat weird setup: first of all, my cable connector was attached the wrong way around, so I always have to use this one in that weird folded-over position.

But a more important issue is that the ISP connection needs to use pins 1..6 of the 2×4-pin SPI/ISP connector on the JeeSMD. That doesn’t work with normal flat cable connectors, which assume 2×3 pins and are too wide to fit in a 2×4-pin header. My solution is to insert wire-wrap pins the wrong way around into the cable header. This effectively extends the connector, but now it won’t be as wide and it’ll fit just fine. Another solution would be to only solder 2×3 pins in the SPI/ISP position – you can always add two more pins later.

Once you’ve passed that hurdle, you can use any ISP programmer you like. There have been several posts about this on the weblog, as listed here.

Now, if you want to use FTDI, then presumably you just uploaded a bootloader into the ATmega, with all the proper fuse settings, etc. The next step then, is to somehow connect to a 6-pin FDTI header.

There are several ways to do this. The one I use nowadays, is through a Carrier Board, which includes the 6-pin FTDI connector:

Dsc 1786

The point about the FTDI connector, is that it’s almost trivial. All you need is 4 wires to GND, PWR, TX, and RX – plus a way to reset the board from the RTS signal. The clever way to generate a reset is to insert a 0.1 µF capacitor between the serial side RTS and the ATmega’s reset pin. Tiny trick, huge implications (does the name “Arduino” ring a bell?).

So how does the Carrier Board implement FTDI? Easy: it adds the capacitor. And you can easily do that yourself without a Carrier Board. Here’s how:

Screen Shot 2010 07 08 at 23.20.52

Note that what FTDI calls “RX” is connected to what the ATmega calls “TXD”, and vice versa. It’s all a matter of perspective… Once you have the FTDI connection set up, you can upload sketches using the Arduino IDE just as with any other board. All you need is a USB-BUB or some other equivalent USB-to-FTDI interface.

Congratulations: that’s all it takes to build and use the Arduino-compatible JeeSMD!