T83C5121 datasheet(65/110 Pages) ATMEL

65 / 110 page

T8xC5121

4164E–SCR–02/04

}

void generate_crc_in_frame(void)

{

checksum_tx=(Uint16) FFFFh;

/* init of the crc variable */

/* loop which compute for each byte (data_byte) to load */

checksum_tx=compute_crc((Uint16)data_byte^checksum_tx)^(checksum_tx>>8);

/* end of loop */

checksum=~checksum_tx;

/* inverts the checksum, so the check will calculate

the CRC of all the datas and */

/* will find a constant value = F0B8

which is the CRC_REF const. of the Bootloader */

write_frame(LOW_BYTE(checksum));

/* writes the LOW_BYTE of the CRC first */

write_frame(HIGH_BYTE(checksum)); /* writes the HIGH_BYTE */

}

***************************************************************************************************

Table 47. Synthesis of Transfer Protection Mechanisms

Notes:

1. The transfer of SSB Byte is also secured by CRC as the CRC is computed on all the

16K data.

2. If a Bad transfer has occurred in the Internal EEPROM (CRC is bad), as the CRC

check is finally done at the end of CRAM programming, application program will NOT

be executed after any Reset.

Read/Write Protection

Lock Byte

In order to protect the content of the internal EEPROM, a Software Security Byte (SSB)

defines two security levels:

•

level 0: SSB = 0xFF: Write and Read are allowed

•

level 1: SSB = 0xFE: Write is disabled

•

level 2: SSB = 0xFC: Write and Read are disabled

This SSB Byte is located at address 3FFDh.

When the level 2 is set, the command to set level 1 is disabled. The security levels can

only be increased.

Source

Target

Check

MCU

CRAM

CRC computed during CRAM Write operation: if error an error code is applied

on P3.7 and Code execution by LJMP000 is not done.

Intern. EEP

MCU

This Read operation is secured by the Write sequence described above

MCU

Intern. EEP

Same protection as in first row above because CRAM is written in sequence

after each page programming of EEP

Ext. EEP

MCU

Same as above as data are transferred to EEP INT and then to CRAM

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