Program and Data
Memory Lock Bits
The AT90S8535 MCU provides two Lock bits that can be left unprogrammed (“1”) or can
be programmed (“0”) to obtain the additional features listed in Table 35. The Lock bits
can only be erased with the Chip Erase command.
1. In Parallel Mode, further programming of the Fuse bits is also disabled. Program the
Fuse bits before programming the Lock bits.
The AT90S8535 has two Fuse bits, SPIEN and FSTRT.
When the SPIEN Fuse is programmed (“0”), Serial Program and Data Downloading
is enabled. Default value is programmed (“0”). The SPIEN Fuse is not accessible in
Serial Programming Mode.
When the FSTRT Fuse is programmed (“0”), the short start-up time is selected.
Default value is unprogrammed (“1”).
The status of the Fuse bits is not affected by Chip Erase.
All Atmel microcontrollers have a three-byte signature code that identifies the device.
This code can be read in both Serial and Parallel modes. The three bytes reside in a
separate address space.
$000: $1E (indicates manufactured by Atmel)
$001: $93 (indicates 8K bytes Flash memory)
$002: $03 (indicates AT90S8535 device when signature byte $001 is $93)
1. When both Lock bits are programmed (lock mode 3), the signature bytes cannot be
read in Serial Mode. Reading the signature bytes will return: $00, $01 and $02.
Programming the Flash
Atmel’s AT90S8535 offers 8K bytes of in-system reprogrammable Flash program mem-
ory and 512 bytes of EEPROM data memory.
The AT90S8535 is shipped with the On-chip Flash program and EEPROM data memory
arrays in the erased state (i.e., contents = $FF) and ready to be programmed. This
device supports a high-voltage (12V) Parallel Programming Mode and a low-voltage
Serial Programming Mode. The +12V is used for programming enable only and no cur-
rent of significance is drawn by this pin. The Serial Programming Mode provides a
convenient way to download program and data into the AT90S8535 inside the user’s
The program and data memory arrays on the AT90S8535 are programmed byte-by-byte
in either programming mode. For the EEPROM, an auto-erase cycle is provided within
the self-timed write instruction in the Serial Programming Mode.
During programming, the supply voltage must be in accordance with Table 36.
Table 35. Lock Bit Protection Modes
Memory Lock Bits
No memory lock features enabled.
Further programming of the Flash and EEPROM is disabled.
Same as mode 2 and verify is also disabled.