Galvanic separation between your target circuit and your PC is desirable sometimes. For some processor families, insulated programming adapters are available at low cost. This is not the case for AVR microprocessors from Atmel. This small circuit adds galvanic separation to any ISP adapter for AVRs.
Every firmware developer always ascertains that all measuring clamps and supply contacts have been removed before flashing a target circuit, of course, so that the target circuit is floating free of potential, before it gets attached to the PC. Absolutely never, a scrub of clamps, probes, soldering iron and cables on the desk confuses the developer's overview. He always bears in mind to separate his target circuit from supply voltage before the programmer gets attached. Therefore, the small circuit presented here, is totally unnecessary.
It's just a small adapter, which allows totally tranparent in-circuit-programming of Atmel-AVRs (such as the famous ATmegas and ATtinys) while being galvanically separated. It can be used transparently with any in-circuit-programmer using the ISP-protocol with up to 2 MHz. DebugWire is not supported.
It consists of the integrated circuit ADUM1401 and some few passive components. It can be either assembled on an adapter PCB for SMD components — or on its own PCB.
The isolator is powered both by the target circuit and the ISP-adapter. Therefore, it works for 5V- and 3,3V-Targets and serves as level-shifter for the latter. This means, if you use the AVRISPmkII, it should be modified to supply the ISP-Isolator. For most other ISPs, only a jumper has to be set. If it is undesirable to modify the ISP, an internal DC/DC-converter can be added to the ISP-Isolator. This feature is prepared in PCB layout which can be downloaded below.
|Schematics and Eagle project files|