ATmega88 datasheet(255/340 Pages) ATMEL

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7530H–AVR–02/09

ATmega48/88/168 Automotive

When designing a system where debugWIRE will be used, the following observations must be

made for correct operation:

• Pull-up resistors on the dW/(RESET) line must not be smaller than 10k

Ω. The pull-up resistor

is not required for debugWIRE functionality.

• Connecting the RESET pin directly to V

CC will not work.

• Capacitors connected to the RESET pin must be disconnected when using debugWire.

• All external reset sources must be disconnected.

22.4

Software Break Points

debugWIRE supports Program memory Break Points by the AVR Break instruction. Setting a

Break Point in AVR Studio

® will insert a BREAK instruction in the Program memory. The instruc-

tion replaced by the BREAK instruction will be stored. When program execution is continued, the

stored instruction will be executed before continuing from the Program memory. A break can be

inserted manually by putting the BREAK instruction in the program.

The Flash must be re-programmed each time a Break Point is changed. This is automatically

handled by AVR Studio through the debugWIRE interface. The use of Break Points will therefore

reduce the Flash Data retention. Devices used for debugging purposes should not be shipped to

end customers.

22.5

Limitations of debugWIRE

The debugWIRE communication pin (dW) is physically located on the same pin as External

Reset (RESET). An External Reset source is therefore not supported when the debugWIRE is

enabled.

The debugWIRE system accurately emulates all I/O functions when running at full speed, i.e.,

when the program in the CPU is running. When the CPU is stopped, care must be taken while

accessing some of the I/O Registers via the debugger (AVR Studio).

A programmed DWEN Fuse enables some parts of the clock system to be running in all sleep

modes. This will increase the power consumption while in sleep. Thus, the DWEN Fuse should

be disabled when debugWire is not used.

22.6

debugWIRE Related Register in I/O Memory

The following section describes the registers used with the debugWire.

22.6.1

debugWire Data Register – DWDR

The DWDR Register provides a communication channel from the running program in the MCU

to the debugger. This register is only accessible by the debugWIRE and can therefore not be

used as a general purpose register in the normal operations.

Bit

76543210

DWDR[7:0]

DWDR

Read/Write

R/W

Initial Value

00000000

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