
23
4428E–8051–02/08
AT/TS80C31X2
12. ONCETM Mode (ON Chip Emulation)
The ONCE mode facilitates testing and debugging of systems using TS80C31X2 without remov-
ing the circuit from the board. The ONCE mode is invoked by driving certain pins of the
TS80C31X2; 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 TS80C31X2 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 12-1.
External Pin Status during ONCE Mode
ALE
PSEN
Port 0
Port 1
Port 2
Port 3
XTAL1/2
Weak pull-up
Float
Weak pull-up
Active