at89c5131 ATMEL Corporation, at89c5131 Datasheet - Page 153

• Manufacturer Part Number: at89c5131
• Description: 8-bit Flash Microcontroller With Full Speed Usb Device At89c5131
• Manufacturer: ATMEL Corporation

ONCE Mode (ON
Chip Emulation)
4136C–USB–04/05
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:
•
•
While the AT89C5131 is in ONCE mode, an emulator or test CPU can be used to drive
the circuit Table 109 shows the status of the port pins during ONCE mode.
Normal operation is restored when normal reset is applied.
Table 109. External Pin Status during ONCE Mode
pull-up
Weak
ALE
Pull ALE low while the device is in reset (RST high) and PSEN is high.
Hold ALE low as RST is deactivated.
PSEN
pull-up
Weak
Port 0
Float
Port 1
pull-up
Weak
Port 2
pull-up
Weak
Port 3
pull-up
Weak
Port I2
Float
AT89C5131
XTALA1/2
Active
XTALB1/2
Active
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