12-7
SPECIAL OPERATING MODES
12.5 ON-CIRCUIT EMULATION (ONCE) MODE
The on-circuit emulation (ONCE) mode permits external testers to test and debug 8XC251Sx-
based systems without removing the chip from the circuit board. A clamp-on emulator or test
CPU is used in place of the 8XC251Sx which is electrically isolated from the system.
12.5.1 Entering ONCE Mode
To enter the ONCE mode:
1. Assert RST to initiate a device reset. See section 11.4.1, “Externally Initiated Resets,” and
the reset waveforms in Figure 11-5 on page 11-8.
2. While holding RST asserted, apply and hold logic levels to I/O pins as follows: PSEN# =
low, P0.7:5 = low, P0.4 = high, P0.3:0 = low (i.e., port 0 = 10H).
3. Deassert RST, then remove the logic levels from PSEN# and port 0.
These actions cause the 8XC251Sx to enter the ONCE mode. Port 1, 2, and 3 pins are weakly
pulled high and port 0, ALE, and PSEN# pins are floating (Table 12-1). Thus the device is elec-
trically isolated from the remainder of the system which can then be tested by an emulator or test
CPU. Note that in the ONCE mode the device oscillator remains active.
12.5.2 Exiting ONCE Mode
To exit ONCE mode, reset the device.
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