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Evil Mad Scientist writes –

This is a quick how-to guide on making ultra-simple development boards for programming AVR microcontrollers. Most recent-vintage AVRs can be programmed through an ISP (in-system programmer) connection; all that is really needed is a place for the chip to sit and a way to connect to the programmer. That’s where these minimalist AVR “target boards” come in. These little boards cost only about $2 and take only a few minutes to make. It turns out that I build a lot of these because asking “how do you make a circuit to program the AVR?” is really the same as asking how you can program an AVR that is in a circuit. And, we might as well start with a simple circuit.

There are two basic types of microcontroller programmer that you can imagine: one that has a socket for your chip, and one that hooks up to your circuit where your chip is installed. We’re interested in the latter, the set of so-called in-system programmers, sometimes called in-circuit programmers. You can also get programmers with dedicated sockets, of course, but they are typically more expensive and are not fundamentally “better.” With an in-system programmer, not only can you program microcontrollers that are in larger circuits, but you can also easily make programmer target boards with sockets, giving you the benefits of a programmer with a socket.

Using AVR microcontrollers: Minimalist target boards – Link.

Related:
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USBtinyISP AVR programmer & SPI interface – Link.