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Tuesday, February 5, 2013

Simple AVR ISP Circuit


Here’s a design circuit for simplest AVR ISP circuit is shown here, with some notes. Ground and VCC must be connected to programmer, either to power programmer or to provide a reference voltage for lower voltage circuits. This is the figure of the circuit;


Most AVR microcontrollers use MOSI, MISO and SCK SPI pins for programming, but some such as ATmega128 use SCK and then UART TX and RX pins instead. The capacitor connected between Reset and Ground, and the resistor from Reset to Vcc should be fitted to give a slight delay to allow AVR to power up properly. These values are not critical, but if they are too large then the ISP programmer will need to be slowed down. The programming lines (SCK, MISO and MOSI) are best left just for programming, but if these pins must be used by the application, then resistors should be used to isolate the application circuitry, typically 4K7. This is especially important if TXD/RXD are used for programming, as UART chips tend to hold the lines. This should be fine for SPI or UART use or where pins are inputs but if you have to use these lines for higher current, then a multiplexer circuit may be needed, see STK200 schematics. Capacitors on the programming lines can cause problems, especially on SCK. If they have to be fitted, then they should be below 10nF and fitted as close as possible to AVR microcontroller pins. Some low cost programmers will have problems with even a 10nF capacitor on SCK or MOSI, so smaller is better.
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