T89C51RD2-3CSCM View Datasheet(PDF) - Atmel Corporation

Part Name

Description

Manufacturer

T89C51RD2-3CSCM

0 to 40MHz Flash Programmable 8-bit Microcontroller

Atmel Corporation 

ONCE(TM) Mode (ON Chip

Emulation)

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:

• Pull ALE low while the device is in reset (RST high) and PSEN is high.

• Hold ALE low as RST is deactivated.

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

ALE

PSEN

Weak pull-up Weak pull-up

Port 0

Float

Port 1

Port 2

Port 3

Weak pull-up Weak pull-up Weak pull-up

XTAL1/2

Active

(a) "Once" is a registered trademark of Intel Corporation.

48 T89C51RD2

4243G–8051–05/03

Share Link: