Filter
Top Products
12-1
CHAPTER 12
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 log-
ic and essentially “take over” operation of the board (without removing the soldered device from
the board).
12.1 ENTERING/LEAVING ONCE MODE
Forcing UCS
and LCS low while RES is asserted (low) enables ONCE Mode (see Figure 12-1).
Maintaining UCS
and LCS and RES low continues to keep ONCE Mode active. Returning UCS
and LCS high exits ONCE Mode.
However, it is possible to keep ONCE Mode always active by deasserting RES
while keeping
UCS
and LCS low. Removing RES “latches” ONCE Mode and allows UCS and LCS to be driv-
en to any level. UCS
and LCS must remain low for at least one clock beyond the time RES is
driven high. Asserting RES
exits ONCE Mode, assuming UCS and LCS do not also remain low
(see Figure 12-1).