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8xC196EA microcontroller user's manual.1998.pdf
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SPECIAL OPERATING MODES

For example, assume that the oscillator needs at least 12.5 ms to stabilize (TDIS = 12.5 ms), Vt is 2.5 V, and the discharge current is 200 μA. The minimum C1 capacitor size is 1 μF.

( 0.0125) (0.0002) μ

C1 = ---------------------------------------------- = 1 F 2.5

When using an external oscillator, the value of C1 can be very small, allowing rapid recovery from powerdown. For example, a 100 pF capacitor discharges in 1.25 μs.

C1

× Vt

=

( 1.0

× 1010)( 2.5)

=

1.25 μs

TDIS = ---------

I---------

-------------

--0.0002-------------------------------

 

 

 

 

 

14.5 ONCE MODE

On-circuit emulation (ONCE) mode isolates the microcontroller from other components in the system to allow printed-circuit-board testing and debugging with a clip-on emulator. During ONCE mode, all pins except XTAL1, XTAL2, RESET#, VSS, and VCC are in a high impedence state. RESET# must be held high; otherwise, the microcontroller will exit ONCE mode and enter the reset state.

Holding the ONCE# signal low during the rising edge of RESET# causes the microcontroller to enter ONCE mode. The PLLEN pin must also be held low. The ONCE signal is latched when RESET goes inactive. Internally, the ONCE# pin is tied to a medium-strength pull-up. To prevent accidental entry into ONCE mode, connect the ONCE# pin to VCC.

Exit ONCE mode by asserting the RESET# signal. Normal operations resume when RESET# goes high.

14-11

15

Interfacing with

External Memory

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