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This chapter describes the i960
Emulation) and Boundary Scan (JTAG). Together these two features create a powerful
environment for design debug and fault diagnosis.
15.1
ON-CIRCUIT EMULATION (ONCE)
On-circuit emulation aids board-level testing. This feature allows a mounted i960 Jx processor to
electrically "remove" itself from a circuit board. This allows for system-level testing where a
remote tester exercises the processor system. In ONCE mode, the processor presents a high
impedance on every pin, except for the JTAG Test Data Output (TDO). All pullup transistors
present on input pins are also disabled and internal clocks stop. In this state the processor's power
demands on the circuit board are nearly eliminated. Once the processor is electrically removed, a
functional tester such as an In-Circuit Emulator (ICE) system can emulate the mounted processor
and execute a test of the i960 Jx processor system.
15.1.1
Entering/Exiting ONCE Mode
The i960 Jx processor uses the dual function LOCK/ONCE pin for ONCE. The LOCK/ONCE pin
is an input while RESET is asserted. The i960 Jx processor uses this pin as an output when the
ONCE mode conditions are not present.
ONCE mode is entered by asserting (low) the LOCK/ONCE pin while the processor is in the reset
state, or by executing the HIGHZ JTAG private instruction. The LOCK/ONCE pin state is latched
on the RESET signal's rising edge.
•
To enter ONCE mode, an external tester drives the ONCE pin low (overcoming the internal
pull-up resistor) and initiates a reset cycle.
•
To exit ONCE mode, perform a hard reset with the ONCE pin deasserted (high) prior to the
rising edge of RESET. It is not necessary to cycle power when exiting ONCE mode.
For specific timing of the LOCK/ONCE pin and the characteristics of the on-circuit emulation
mode, see related documents in
®
Jx processor's test features, including ONCE (On-Circuit
section 1.4, "Related Documents" (pg.
CHAPTER 15
TEST FEATURES
1-10).
15
15-1
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