Rainbow Electronics AT89C5131 - Manual (Page 150)

150
AT89C5131
4136A–USB–03/03
ONCE Mode (ON
Chip Emulation)
The ONCE mode facilitates testing and debugging of systems using AT89C5131 without
removing the circuit from the board. The ONCE mode is invoked by driving certain pins
of the AT89C5131; 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 AT89C5131 is in ONCE mode, an emulator or test CPU can be used to drive
the circuit Table 102 shows the status of the port pins during ONCE mode.
Normal operation is restored when normal reset is applied.
Table 102. External Pin Status during ONCE Mode
ALE PSEN Port 0 Port 1 Port 2 Port 3 Port I2 XTALA1/2 XTALB1/2
Weak
pull-up
Weak
pull-up
Float
Weak
pull-up
Weak
pull-up
Weak
pull-up
Float Active Active