Accessing 16-Bit Registers
The Microchip AVR peripherals typically contain several 16-bit registers that can be accessed via the 8-bit data bus
(e.g., TCNTn of a 16-bit timer). The 16-bit register must be byte accessed using two read or write operations.
Breaking in the middle of a 16-bit access or single-stepping through this situation may result in erroneous values.
Restricted I/O Register Access
Certain registers cannot be read without affecting their contents. Such registers include those which contain flags
which are cleared by reading, or buffered data registers (e.g., UDR). The software front-end will prevent reading
these registers when in Stopped mode to preserve the intended non-intrusive nature of OCD debugging. In addition,
some registers cannot safely be written without side-effects occurring - these registers are read-only. For example:
• Flag registers, where a flag is cleared by writing ‘1’ to any bit. These registers are read-only.
• UDR and SPDR registers cannot be read without affecting the state of the module. These registers are not
accessible.
4.4.12
megaAVR Special Considerations
Software Breakpoints
Since it contains an early version of the 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. 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 V
CC
changes. Be conservative when specifying the target clock frequency.
OCDEN Fuse
To be able to debug a megaAVR device the OCDEN fuse must be programmed (by default OCDEN is
unprogrammed). 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 programmend when starting a debug session and is left unprogrammed
when terminating the session, thereby restricting unnecessary power consumption by the OCD module.
JTAGEN Fuse
The JTAG interface is enabled using the JTAGEN fuse, which is programmed by default. This allows access to the
JTAG programming interface.
Important:
If the JTAGEN fuse is unintentionally disabled, it can only be re-enabled using SPI or High
Voltage programming methods.
If the JTAGEN fuse is programmed, the JTAG interface can still be disabled in firmware by setting the JTAG disable
bit in the MCU Control Register. This will render code un-debuggable, and should not be done when attempting a
debug session. If such code is already executing on the Microchip AVR device when starting a debug session, the
Power Debugger 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 JTAG disable bit from the program
code, or by clearing the JTAGEN fuse through a programming interface.
Power Debugger
On-chip Debugging
©
2020 Microchip Technology Inc.
User Guide
DS40002201A-page 71
