12-12
8XC196NP, 80C196NU USER’S MANUAL
For example, assume that the oscillator needs at least 12.5 ms to discharge (T
DIS
= 12.5 ms), V
t
is 2.5 V, and the discharge current is 200 µA. The minimum C
1
capacitor size is 1 µF.
When using an external oscillator, the value of C
1
can be very small, allowing rapid recovery from
powerdown. For example, a 100 pF capacitor discharges in 1.25 µs.
12.6 ONCE MODE
On-circuit emulation (ONCE) mode isolates the device from other components in the system to
allow printed-circuit-board testing or debugging with a clip-on emulator. During ONCE mode,
all pins except XTAL1, XTAL2, V
SS
, and V
CC
are weakly pulled high or low. During ONCE
mode, RESET# must be held high or the device will exit ONCE mode and enter the reset state.
Holding the ONCE signal high during the rising edge of RESET# causes the device to enter
ONCE mode. The ONCE signal is latched when RESET# goes inactive. Internally, the ONCE pin
is tied to a medium-strength pull-down. To prevent accidental entry into ONCE mode, connect
the ONCE pin to V
SS
.
Exit ONCE mode by asserting the RESET# signal. Normal operations resume when RESET#
goes high.
12.7 RESERVED TEST MODES (80C196NU ONLY)
For the 80C196NU only, holding PLLEN1 low while PLLEN2 is held high causes the device to
enter an unsupported test mode. Table 12-3 shows the proper PLLEN1 and PLLEN2 connections
for valid clock modes.
C
1
0.0125()0.0002()
2.5
--------------------------------------------------
1 µF==
T
DIS
C
1
V
t
×
I
------------------
1.0 10
10–
×()2.5()
0.0002
---------------------------------------------------
1.25 µs== =