TS80C51RA2 Atmel Corporation, TS80C51RA2 Datasheet - Page 47

• Manufacturer Part Number: TS80C51RA2
• Description: High Performance 8-bit Microcontroller
• Manufacturer: Atmel Corporation

ONCE
Emulation)
4188A–8051–10/02
TM
Mode (ON Chip
The ONCE mode facilitates testing and debugging of systems using TS8xC51Rx2 with-
out removing the circuit from the board. The ONCE mode is invoked by driving certain
pins of the TS80C51Rx2; the following sequence must be exercised:
•
•
While the TS80C51Rx2 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 27. External Pin Status during ONCE Mode
Weak pull-
Pull ALE low while the device is in reset (RST high) and PSEN is high.
Hold ALE low as RST is deactivated.
ALE
up
Weak pull-
PSEN
up
Port 0
Float
Weak pull-
Port 1
up
Weak pull-
Port 2
up
TS8xC51Rx2
Weak pull-
Port 3
up
XTAL1/2
Active
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