P89C668 datasheet(44/54 Pages) PHILIPS

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Philips Semiconductors

Preliminary data

P89C668

80C51 8-bit Flash microcontroller family

64KB ISP Flash with 8KB RAM

2001 Jul 27

FFFF

C000

8000

4000

2000

0000

BLOCK 4

16k BYTES

BLOCK 3

16k BYTES

BLOCK 2

16k BYTES

BLOCK 1

8k BYTES

BLOCK 0

8k BYTES

PROGRAM

ADDRESS

BOOT ROM

(1k BYTES)

FFFF

FC00

SU01112

Figure 45. Flash Memory Configurations

Power-On Reset Code Execution

The P89C668 contains two special Flash registers: the BOOT

VECTOR and the STATUS BYTE. At the falling edge of reset, the

P89C668 examines the contents of the Status Byte. If the Status

Byte is set to zero, power-up execution starts at location 0000H,

which is the normal start address of the user’s application code.

When the Status Byte is set to a value other than zero, the contents

of the Boot Vector is used as the high byte of the execution address

and the low byte is set to 00H. The factory default setting is 0FCH,

corresponds to the address 0FC00H for the factory masked-ROM

ISP boot loader. A custom boot loader can be written with the Boot

Vector set to the custom boot loader.

NOTE:

When erasing the Status Byte or Boot Vector,

both bytes are erased at the same time. It is necessary

to reprogram the Boot Vector after erasing and

updating the Status Byte.

Hardware Activation of the Boot Loader

The boot loader can also be executed by holding PSEN LOW, P2.7

high, EA greater than VIH (such as +5 V), and ALE HIGH (or not

connected) at the falling edge of RESET. This is the same effect as

having a non-zero status byte. This allows an application to be built

that will normally execute the end user’s code but can be manually

forced into ISP operation.

If the factory default setting for the Boot Vector (0FCH) is changed, it

will no longer point to the ISP masked-ROM boot loader code. If this

happens, the only way it is possible to change the contents of the

Boot Vector is through the parallel programming method, provided

that the end user application does not contain a customized loader

that provides for erasing and reprogramming of the Boot Vector and

Status Byte.

After programming the Flash, the status byte should be programmed

to zero in order to allow execution of the user’s application code

beginning at address 0000H.

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