This week, Brad wrote in with an interesting question: Can you program the ATtiny25 on one of our ‘tiny2313 target boards? And the answer is yes: you can, with just a trivial modification.

Okay, back up– a little context here. The ATtiny25 and the ATtiny2313 are examples of AVR microcontrollers, the little brains that power many of our projects.
To program these chips, we use a usbtinyisp programmer, hooked up to a minimalist target board.

The target board basically provides a programming header that’s hooked up to the right pins of the chip, plus some way to power the chip– often through the programmer itself.

After hand-wiring up one too many minimalist ‘2313 boards like that, we also made a printed circuit board version of the ‘2313 target board. Normally, it looks something like this, with an ATtiny2313 in a ZIF socket:

But, back to the question. The ATtiny25, ATtiny45, and ATtiny85 are a family of 8-pin AVR microcontrollers that are not pin compatible with the ‘2313. However, at only 8 pins instead of 20 pins, they’ll definitely fit in the socket… somewhere.

Looking at the datasheets and pinouts for the the ’45, ‘2313 (and the ‘168 that we also have a target board for), we can identify the lines used for programming: MISO, MOSI, SCK, and RESET:

 
The chips also need power and ground connections to be programmed, of course. Now if you notice, the connections for the ‘2313 and ’45 are very similar– in fact, almost identical if you line the chip up so that pin 1 goes where pin 1 of the ‘2313 normally would. The one remaining difference is that there’s no ground connection to pin 4 of the ‘2313.

So, adding a wire from ground– pin 10 of the ‘2313 –to pin 4 of the smaller chip (an ATtiny25 in our photo), and lining up pin 1 to pin 1, we’re ready to go. And yes, it works like a charm.
If you do use this method, there are a few (possibly obvious) things worth noting:

 

• You need to be careful to line up pin 1 of the chip to pin 1 of the socket.
• Be careful if or when you put a ‘2313 chip back in the socket. It will draw a lot of current if you set pin 4’s to a high output level– it’s shorted to ground. (Better: If you want to go back and forth, use a switch, not a wire.)
• Keep in mind that the pin labels on the target board are for the ‘2313, not the ’85.

Going one step further, you could also potentially program the ’25/’45/’85 from the ‘168 target board: it only takes a couple more wires. To do so, line up pin 1 of the ’25 to pin 9 of the ‘168. Add two wires this time, from board-reset to chip-reset, and from board-ground to chip-ground. You’ll also need to connect AVCC (analog power supply) to VCC. A little more work, yes, but still a good hack.