80C186EC/80C188EC Microprocessor User's Manual

CHAPTER 15
ONCE MODE
ONCE (pronounced “ahnce”) Mode provides the ability to three-state all output, bidirectional, or
weakly held high/low pins except OSCOUT. To allow device operation with a crystal network,
OSCOUT does not three-state.
ONCE Mode electrically isolates the device from the rest of the board logic. This isolation allows
a bed-of-nails tester to drive the device pins directly for more accurate and thorough testing. An
in-circuit emulation probe uses ONCE Mode to isolate a surface-mounted device from board logic
and essentially “take over” operation of the board (without removing the soldered device from
the board).
15.1 ENTERING/LEAVING ONCE MODE
Forcing A19/ONCE low while RESIN is asserted (low) enables ONCE Mode (see Figure 15-1).
Maintaining A19/ONCE and RESIN low continues to keep ONCE Mode active. Returning
A19/ONCE high exits ONCE Mode.
However, it is possible to keep ONCE Mode always active by deasserting RESIN while keeping
A19/ONCE low. Removing RESIN “latches” ONCE Mode and allows A19/ONCE to be driven
to any level. A19/ONCE must remain low for at least one clock beyond the time RESIN is driven
high. Asserting RESIN exits ONCE Mode, assuming A19/ONCE does not also remain low (see
Figure 15-1).
RESIN
2
A19/ONCE
All output,
bidirectional,
weakly held
pins except
OSCOUT
1
3
NOTES:
1. Entering ONCE Mode.
2. Latching ONCE Mode.
3. Leaving ONCE Mode (assuming 2 occurred).
A1260-0A
Figure 15-1. Entering/Leaving ONCE Mode
15-1