Rainbow Electronics ATmega128L - Manual (Page 243)

243
ATmega128(L)
2467B–09/01
to this location. At the same time, an internal flag; I/O Debug Register Dirty – IDRD – is
set to indicate to the debugger that the register has been written. When the CPU reads
the OCDR register the 7 LSB will be from the OCDR register, while the MSB is the IDRD
bit. The debugger clears the IDRD bit when it has read the information.
In some AVR devices, this register is shared with a standard I/O location. In this case,
the OCDR register can only be accessed if the OCDEN fuse is programmed, and the
debugger enables access to the OCDR register. In all other cases, the standard I/O
location is accessed.
Refer to the debugger documentation for further information on how to use this register.
Using the JTAG
Programming
Capabilities Programming of AVR parts via JTAG is performed via the four-pin JTAG port, TCK,
TMS, TDI and TDO. These are the only pins that need to be controlled/observed to per-
form JTAG programming (in addition to power pins). It is not required to apply 12V
externally. The JTAGEN fuse must be programmed and the JTD bit in the MCUSR reg-
ister must be cleared to enable the JTAG Test Access Port.
The JTAG programming capability supports:
• Flash programming and verifying
• EEPROM programming and verifying
• Fuse programming and verifying
• Lock bit programming and verifying
The lock bit security is exactly as in parallel programming mode. If the lock-bits LB1 or
LB2 are programmed, the OCDEN Fuse cannot be programmed unless first doing a
chip erase. This is a security feature that ensures no back-door exists for reading out the
content of a secured device.
The details on programming through the JTAG interface and programming specific
JTAG instructions are given in the section
“Programming Via the JTAG Interface” on
page 297.
Bibliography For more information about general Boundary-scan, the following literature can be
consulted:
• IEEE: IEEE Std 1149.1-1990. IEEE Standard Test Access Port and Boundary-scan
Architecture, IEEE, 1993
• Colin Maunder: The Board Designers Guide to Testable Logic Circuits, Addison-
Wesley, 1992