TS80C54X2 Datasheet, PDF (32/58 Pages) ATMEL Corporation – 8-bit CMOS Microcontroller 16/32 Kbytes ROM/OTP

TS80C54X2/C58X2
TS87C54X2/C58X2
6.9 ONCE^TM Mode (ON Chip Emulation)
The ONCE mode facilitates testing and debugging of systems using TS80C54/58X2 without removing the circuit
from the board. The ONCE mode is invoked by driving certain pins of the TS80C54/58X2; the following sequence
must be exercised:
q Pull ALE low while the device is in reset (RST high) and PSEN is high.
q Hold ALE low as RST is deactivated.
While the TS80C54/58X2 is in ONCE mode, an emulator or test CPU can be used to drive the circuit Table 26.
shows the status of the port pins during ONCE mode.
Normal operation is restored when normal reset is applied.
Table 17. External Pin Status during ONCE Mode
ALE
PSEN
Port 0
Port 1
Port 2
Port 3
XTAL1/2
Weak pull-up
Weak pull-up
Float
Weak pull-up
Weak pull-up
Weak pull-up
Active
32
Rev. B - Aug. 31, 1999