P89C668 Philips Semiconductors, P89C668 Datasheet - Page 44

• Manufacturer Part Number: P89C668
• Description: 80C51 8-bit Flash microcontroller family 64KB ISP FLASH with 8KB RAM
• Manufacturer: Philips Semiconductors

Philips Semiconductors
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.
2001 Jul 27
80C51 8-bit Flash microcontroller family
64KB ISP Flash with 8KB RAM
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.
PROGRAM
ADDRESS
Figure 45. Flash Memory Configurations
FFFF
C000
8000
4000
2000
0000
16k BYTES
16k BYTES
16k BYTES
8k BYTES
8k BYTES
BLOCK 4
BLOCK 3
BLOCK 2
BLOCK 1
BLOCK 0
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If the factory default setting for the Boot Vector (0FCH) is changed, it
After programming the Flash, the status byte should be programmed
Hardware Activation of the Boot Loader
The boot loader can also be executed by holding PSEN LOW, P2.7
high, EA greater than V
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.
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.
to zero in order to allow execution of the user’s application code
beginning at address 0000H.
BOOT ROM
(1k BYTES)
IH
FFFF
FC00
(such as +5 V), and ALE HIGH (or not
SU01112
P89C668
Preliminary data