t89c51rd2-slscl ATMEL Corporation, t89c51rd2-slscl Datasheet - Page 48

• Manufacturer Part Number: t89c51rd2-slscl
• Description: 0 To 40 Mhz Flash Programmable 8-bit Microcontroller
• Manufacturer: ATMEL Corporation

ONCE
Emulation)
48
(TM)
T89C51RD2
Mode (ON Chip
The ONCE mode facilitates testing and debugging of systems using T89C51RD2 with-
out removing the circuit from the board. The ONCE mode is invoked by driving certain
pins of the T89C51RD2; the following sequence must be exercised:
•
•
While the T89C51RD2 is in ONCE mode, an emulator or test CPU can be used to drive
the circuit. Table 30. shows the status of the port pins during ONCE mode.
Normal operation is restored when normal reset is applied.
Table 30. External Pin Status during ONCE Mode
(a) "Once" is a registered trademark of Intel Corporation.
Weak pull-up
Pull ALE low while the device is in reset (RST high) and PSEN is high.
Hold ALE low as RST is deactivated.
ALE
Weak pull-up
PSEN
Port 0
Float
Weak pull-up
Port 1
Weak pull-up
Port 2
Weak pull-up
Port 3
4243G–8051–05/03
XTAL1/2
Active