235
ATmega169V/L
2514A–AVR–08/02
Boundary-scan Related
Register in I/O Memory
MCU Control Register –
MCUCR
The MCU Control Register contains control bits for general MCU functions.
• Bits 7 – JTD: JTAG Interface Disable
When this bit is zero, the JTAG interface is enabled if the JTAGEN Fuse is programmed.
If this bit is one, the JTAG interface is disabled. In order to avoid unintentional disabling
or enabling of the JTAG interface, a timed sequence must be followed when changing
this bit: The application software must write this bit to the desired value twice within four
cycles to change its value. Note that this bit must not be altered when using the On-chip
Debug system.
MCU Status Register –
MCUSR
The MCU Status Register provides information on which reset source caused an MCU
reset.
• Bit 4 – JTRF: JTAG Reset Flag
This bit is set if a reset is being caused by a logic one in the JTAG Reset Register
selected by the JTAG instruction AVR_RESET. This bit is reset by a Power-on Reset, or
by writing a logic zero to the flag.
Boundary-scan Chain The Boundary-scan chain has the capability of driving and observing the logic levels on
the digital I/O pins, as well as the boundary between digital and analog logic for analog
circuitry having off-chip connection.
Scanning the Digital Port Pins Figure 108 shows the Boundary-scan Cell for a bi-directional port pin with pull-up func-
tion. The cell consists of a standard Boundary-scan cell for the Pull-up Enable – PUExn
– function, and a bi-directional pin cell that combines the three signals Output Control –
OCxn, Output Data – ODxn, and Input Data – IDxn, into only a two-stage Shift Register.
The port and pin indexes are not used in the following description
The Boundary-scan logic is not included in the figures in the Data Sheet. Figure 109
shows a simple digital port pin as described in the section “I/O-Ports” on page 50. The
Boundary-scan details from Figure 108 replaces the dashed box in Figure 109.
When no alternate port function is present, the Input Data – ID – corresponds to the
PINxn Register value (but ID has no synchronizer), Output Data corresponds to the
PORT Register, Output Control corresponds to the Data Direction – DD Register, and
the Pull-up Enable – PUExn – corresponds to logic expression PUD
· DDxn · PORTxn.
Digital alternate port functions are connected outside the dotted box in Figure 109 to
make the scan chain read the actual pin value. For Analog function, there is a direct
connection from the external pin to the analog circuit, and a scan chain is inserted on
the interface between the digital logic and the analog circuitry.
Bit 76543210
JTD
– – PUD – – IVSEL IVCE MCUCR
Read/Write R/W R R R/W R R R/W R/W
InitialValue00000000
Bit 76543210
– – – JTRF WDRF BORF EXTRF PORF MCUSR
Read/Write R R R R/W R/W R/W R/W R/W
Initial Value 0 0 0 See Bit Description
(27 pages)
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