Rainbow Electronics ATmega128RFA1 - Manual (Page 436)

436
8266A-MCU Wireless-12/09
ATmega128RFA1
Figure 28-1. Block Diagram
TAP
CONTROLLER
TDI
TDO
TCK
TMS
FLASH
MEMORY
AVR CPU
DIGITAL
PERIPHERAL
UNITS
JTAG / AVR CORE
COMMUNICATION
INTERFACE
BREAKPOINT
UNIT
FLOW CONTROL
UNIT
OCD STATUS
AND CONTROL
INTERNAL
SCAN
CHAIN
M
U
X
INSTRUCTION
REGISTER
ID
REGISTER
BYPASS
REGISTER
JTAG PROGRAMMING
INTERFACE
PC
Instruction
Address
Data
BREAKPOINT
SCAN CHAIN
ADDRESS
DECODER
ANALOG
PERIPHERIAL
UNITS
I/O PORT 0
I/O PORT n
BOUNDARY SCAN CHAIN
Analog inputs
Control & Clock lines
DEVICE BOUNDARY
28.3 TAP - Test Access Port
The JTAG interface is accessed through four of the AVR’s pins. In JTAG terminology,
these pins constitute the Test Access Port – TAP. These pins are:
• TMS: Test mode select. This pin is used for navigating through the TAP-controller
state machine.
• TCK: Test Clock. JTAG operation is synchronous to TCK.
• TDI: Test Data In. Serial input data to be shifted in to the Instruction Register or Data
Register (Scan Chains).
• TDO: Test Data Out. Serial output data from Instruction Register or Data Register.
The IEEE std. 1149.1 also specifies an optional TAP signal; TRST – Test ReSeT –
which is not provided.
When the JTAGEN Fuse is un-programmed, these four TAP pins are normal port pins,
and the TAP controller is in reset. When programmed the input TAP signals are
internally pulled high and the JTAG is enabled for Boundary-scan and programming.
The device is shipped with this fuse programmed.
For the on-chip debug system, in addition to the JTAG interface pins, the RESET pin is
monitored by the debugger to be able to detect external reset sources. The debugger
can also pull the RESET pin low to reset the whole system, assuming only open
collectors on the reset line are used in the application.