24

Rev. C - 15 January, 2001

TS80C31X2

The ONCE mode facilitates testing and debugging of systems using TS80C31X2 without removing the circuit from

the board. The ONCE mode is invoked by driving certain pins of the TS80C31X2; 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 TS80C31X2 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 12. 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