ATTINY1634_14 Datasheet, PDF (189/259 Pages) ATMEL Corporation – 8-bit AVR Microcontroller with 16KB In-system

21.4
Software Break Points
debugWIRE supports program memory break points by the AVR^® break instruction. Setting a break point in AVR Studio^®
inserts a BREAK instruction in the program memory. The instruction replaced by the BREAK instruction is stored. When
program execution is continued, the stored instruction is 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 reprogrammed each time a break point is changed. This is automatically handled by AVR Studio through
the debugWIRE interface. The use of break points thus reduces the Flash data retention. Devices used for debugging
purposes should not be shipped to end customers.
21.5
Limitations of debugWIRE
The debugWIRE communication pin (dW) is physically located on the same pin as the 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). See the debugWIRE documentation for more information about the limitations.
The debugWIRE interface is asynchronous, which means that the debugger needs to synchronize to the system clock. If the
system clock is changed by software (e.g., by writing CLKPS bits), communication via debugWIRE may fail. In addition,
clock frequencies below 100kHz may cause communication problems.
A programmed DWEN fuse enables some parts of the clock system to run in all sleep modes. This increases the power
consumption while in sleep. The DWEN fuse should therefore be disabled when debugWIRE is not used.
21.6
Register Description
The following section describes the registers used with the debugWIRE.
21.6.1 DWDR – debugWIRE Data Register
Bit
7
6
5
4
3
2
1
0
0x2E (0x4E)
DWDR[7:0]
DWDR
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
The DWDR register provides a communication channel from the running program in the MCU to the debugger. This register
is only accessible via the debugWIRE and therefore cannot be used as a general purpose register in normal operations.
ATtiny1634 [PRELIMINARY DATASHEET] 189
9296C–AVR–07/14