TS80C51RA2_14 Datasheet, PDF (48/86 Pages) ATMEL Corporation – Programmable Clock Out and Up/Down Timer/Counter 2

6.9 ONCE^TM Mode (ON Chip Emulation)
The ONCE mode facilitates testing and debugging of systems using TS8xC51Rx2 without
removing the circuit from the board. The ONCE mode is invoked by driving certain pins of the
TS80C51Rx2; the following sequence must be exercised:
• Pull ALE low while the device is in reset (RST high) and PSEN is high.
• Hold ALE low as RST is deactivated.
While the TS80C51Rx2 is in ONCE mode, an emulator or test CPU can be used to drive the cir-
cuit Table 26. shows the status of the port pins during ONCE mode.
Normal operation is restored when normal reset is applied.
Table 6-23. External Pin Status during ONCE Mode
ALE
Weak pull-up
PSEN
Weak pull-up
Port 0
Float
Port 1
Weak pull-up
Port 2
Weak pull-up
Port 3
Weak pull-up
XTAL1/2
Active
48 AT/TS8xC51Rx2
4188F–8051–01/08