AVR ISP Circuit Schematics

Posted on Feb 6, 2014

It is important to layout your PCB correctly to allow trouble free In System Programming of AVR microcontrollers. This guide covers the usual problems that are encountered and typical AVR ISP circuit schematics. This guide covers Serial Programming, which is commonly called ISP, not JTAG programming which uses a different connector and algorithm.JTAG interface is used for debugging on larger

AVR ISP Circuit Schematics
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AVR devices (40-pin plus) and can be used for programming as well. Most Kanda AVR programmers also support JTAG. Please see below for AVR JTAG interface. In System Programmers have different amounts of "drive" depending on their price and design, so for maximum flexibility in choosing a programmer, err on the side of caution unless you have compelling reasons for fitting large capacitors and low value resistors. The AVR ISP connector can be either 6 or 10-way, but this makes no difference to your circuit layout. 10-way connectors have more Ground lines to improve noise immunity during programming. The 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. 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...

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