8.3 Atmel megaAVR OCD (JTAG)

Software breakpoints

Since it contains an early OCD module, ATmega128[A] does not support the use of the BREAK instruction for software breakpoints.

JTAG clock

The target clock frequency must be accurately specified in the software front-end before starting a debug session. For synchronization reasons, the JTAG TCK signal must be less than one fourth of the target clock frequency for reliable debugging. Setting the target clock frequency too high will cause failure of a debug session shortly after programming completes. This may be accompanied by several spurious SLEEP, WAKEUP, or IDR messages being displayed. When programming via the JTAG interface, the TCK frequency is limited by the maximum frequency rating of the target device, and not the actual clock frequency being used.

When using the internal RC oscillator, be aware that the frequency may vary from device to device and is affected by temperature and VCC changes. Be conservative when specifying the target clock frequency.

JTAGEN and OCDEN fuses

The JTAG interface is enabled using the JTAGEN fuse, which is programmed by default. This allows access to the JTAG programming interface. Through this mechanism, the OCDEN fuse can be programmed (by default OCDEN is un-programmed). This allows access to the OCD in order to facilitate debugging the device. The software front-end will always ensure that the OCDEN fuse is left un-programmed when terminating a session, thereby restricting unnecessary power consumption by the OCD module. If the JTAGEN fuse is unintentionally disabled, it can only be re-enabled using SPI or PP programming methods.

If the JTAGEN fuse is programmed, the JTAG interface can still be disabled in firmware by setting the JTD bit. This will render code un-debuggable, and should not be done when attempting a debug session. If such code is already executing on the Atmel AVR device when starting a debug session, the AVR Dragon will assert the RESET line while connecting. If this line is wired correctly, it will force the target AVR device into reset, thereby allowing a JTAG connection.

If the JTAG interface is enabled, the JTAG pins cannot be used for alternative pin functions. They will remain dedicated JTAG pins until either the JTAG interface is disabled by setting the JTD bit from the program code, or by clearing the JTAGEN fuse through a programming interface.

IDR events

When the application program writes a byte of data to the OCDR register of the AVR device being debugged, the AVR Dragon reads this value out and displays it in the message window of the software front-end. The IDR registers is polled every 100ms, so writing to it at a higher frequency will NOT yield reliable results. When the AVR device loses power while it is being debugged, spurious IDR events may be reported. This happens because the AVR Dragon may still poll the device as the target voltage drops below the AVR’s minimum operating voltage.