T89C51RD2-3CWC-M datasheet(45/86 Pages) ATMEL

Part #	T89C51RD2-3CWC-M

Description	0 to 40MHz Flash Programmable 8-bit Microcontroller

PDF	86 Pages

Scroll/Zoom	100%

Manufacturer	ATMEL [ATMEL Corporation]

Direct Link	http://www.atmel.com

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45 / 86 page

Rev. F - 15 February, 2001

T89C51RD2

6.11. ONCE(TM) Mode (ON Chip Emulation)

The ONCE mode facilitates testing and debugging of systems using T89C51RD2 without 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 23.

shows the status of the port pins during ONCE mode.

Normal operation is restored when normal reset is applied.

Table 23. External Pin Status during ONCE Mode

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

ALE

PSEN

Port 0

Port 1

Port 2

Port 3

XTAL1/2

Weak pull-up

Float

Weak pull-up

Active

T89C51RD2-3CWC-M Datasheet(PDF) 45 Page - ATMEL Corporation