DS87C520 datasheet(25/45 Pages) DALLAS

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DS87C520/DS83C520 EPROM/ROM High-Speed Microcontrollers

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TIMED-ACCESS PROTECTION

It is useful to protect certain SFR bits from an accidental write operation. The Timed Access procedure

stops an errant CPU from accidentally changing these bits. It requires that the following instructions

precede a write of a protected bit.

MOV

0C7h,

#0Aah

MOV

0C7h,

#55h

Writing an AAh then a 55h to the Timed Access register (location C7h) opens a 3-cycle window for write

access. The window allows software to modify a protected bit(s). If these instructions do not immediately

precede the write operation, then the write will not take effect. The protected bits are:

EXIF.0

BGS

Bandgap Select

WDCON.6

POR

Power-On Reset flag

WDCON.1

EWT

Enable Watchdog Reset

WDCON.0

RWT

Restart Watchdog

WDCON.3

WDIF

Watchdog Interrupt Flag

ROMSIZE.2

RMS2

ROM Size Select 2

ROMSIZE.1

RMS1

ROM Size Select 1

ROMSIZE.0

RMS0

ROM Size Select 0

EPROM PROGRAMMING

The DS87C520 follows standards for a 16kB EPROM version in the 8051 family. It is available in a UV-

erasable, ceramic-windowed package and in plastic packages for one-time user-programmable versions.

The part has unique signature information so programmers can support its specific EPROM options.

ROM-specific features are described later in this data sheet.

Most commercially available device programmers will directly support Dallas Semiconductor

microcontrollers. If your programmer does not, please contact the manufacturer for updated software.

PROGRAMMING PROCEDURE

The DS87C520 should run from a clock speed between 4MHz and 6MHz when being programmed. The

programming fixture should apply address information for each byte to the address lines and the data

value to the data lines. The control signals must be manipulated as shown in Table 9. The diagram in

Table 5 shows the expected electrical connection for programming. Note that the programmer must apply

addresses in demultiplexed fashion to Ports 1 and 2 with data on Port 0. Waveforms and timing are

provided in the Electrical Specifications section.

Program the DS87C520 as follows:

1) Apply the address value,

2) Apply the data value,

3) Select the programming option from Table 9 using the control signals,

4) Increase the voltage on VPP from 5V to 12.75V if writing to the EPROM,

5) Pulse the PROG signal five times for EPROM array and 25 times for encryption table, lock bits, and

other EPROM bits,

6) Repeat as many times as necessary.

DS87C520 Datasheet(PDF) 25 Page - Dallas Semiconductor

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