Datasheet

337
7593L–AVR–09/12
AT90USB64/128
28.5.2 MCUSR – MCU Status Register
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.
28.6 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.
28.6.1 Scanning the digital port pins
Figure 28-3 on page 338 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 sig-
nals 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 28-4 on page 339
shows a simple digital port pin as described in the section “I/O-ports” on page 71. The Boundary-
scan details from Figure 28-3 on page 338 replaces the dashed box in Figure 28-4 on page 339.
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 28-4 on page 339
to make the scan chain read the actual pin value. For analog function, there is a direct connec-
tion 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 driving 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 76543210
– – –JTRFWDRF 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