ATmega16U4 datasheet(325/408 Pages) ATMEL

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325

7766D–AVR–01/09

ATmega16U4/ATmega32U4

26.5.2

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.

26.6

Boundary-scan Chain

The Boundary-scan chain has the capability of driving and observing the logic levels on the digi-

tal I/O pins, as well as the boundary between digital and analog logic for analog circuitry having

off-chip connection.

26.6.1

Scanning the Digital Port Pins

Figure 26-3 shows the Boundary-scan Cell for a bi-directional port pin. The pull-up function is

disabled during Boundary-scan when the JTAG IC contains EXTEST or SAMPLE_PRELOAD.

The cell consists of 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 datasheet. Figure 26-4 shows a

simple digital port pin as described in the section “I/O-Ports” on page 64. The Boundary-scan

details from Figure 26-3 replaces the dashed box in Figure 26-4.

When no alternate port function is present, the Input Data - ID - corresponds to the PINxn Regis-

ter 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 - cor-

responds to logic expression PUD · DDxn · PORTxn.

Digital alternate port functions are connected outside the dotted box in Figure 26-4 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. There is no scan chain on the interface between the digital and

the analog circuitry, but some digital control signal to analog circuitry are turned off to avoid driv-

ing contention on the pads.

When JTAG IR contains EXTEST or SAMPLE_PRELOAD the clock is not sent out on the port

pins even if the CKOUT fuse is programmed. Even though the clock is output when the JTAG IR

contains SAMPLE_PRELOAD, the clock is not sampled by the boundary scan.

Bit

765

210

–

–JTRF

WDRF

BORF

EXTRF

PORF

MCUSR

Read/Write

R/W

Initial Value

See Bit Description

ATmega16U4 Datasheet(PDF) 325 Page - ATMEL Corporation

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