8-bit Microcontroller

Application Note

Rev. 2537C–AVR–01/04

AVR088: Migrating between ATmega8535 and ATmega16

IntroductionThis application note is a guide to help current ATmega8535 users convert existingdesigns to ATmega16. The information given will also help users migrating fromATmega16 to ATmega8535. AT90S8535 users should also read the application noteAVR086: Replacing A90S8535 by ATmega8535.

In addition to the differences described in this document, the following features areavailable on ATmega16:

• JTAG Interface (enabled by default)

The electrical characteristics of the two devices are also different. Check the datasheets for detailed information.

Memory SizesSome memories are bigger in the ATmega16. Table 1 is a comparision of the individ-ual memories.

The Boot Loader area is also changed. The following must be considered:

• Flash page size is 64 words instead of 32 words.

• The No-Read-While-Write section starts at word address 0x1C00 instead of0xC00.

Table 1. Memory Sizes

ATmega8535 ATmega16

Flash 8k bytes 16k bytes

RAM 512 bytes 1k bytes

EEPROM 512 bytes 512 bytes

Interrupt Vectors and Relative Jumps/Calls

The ATmega8535 uses 1-word Interrupt Vectors while the ATmega16 uses 2-wordInterrupt Vectors. 1-word Vectors can only contain RJMP instructions, which can notreach the entire 8k word memory range of ATmega16.

Since RJMP/RCALL can only jump 2k words in any direction, it wraps around the startor end of the 4k word Flash of the ATmega8535 to reach the entire memory span. Wrap-ping RJMP/RCALLs must be changed to JMP/CALLs when using the 8k wordATmega16.

The compiler or assembler will take care of most of these differences. It will either usethe correct instruction or issue an error message.

The Interrupt tables are equal.

Fuse Bits Two of the Fuse bits are different. Table 2 shows the Fuse bits.

Miscellaneous The following applies to ATmega16:

• Watchdog Safety Levels not implemented, meaning that the prescaler settings can be changed without the timed sequence used with ATmega8535.

• Watchdog Always On Mode is not implemented.

• Note that if the On-chip Debug System is enabled, the main clock will continue running in all sleep modes. This will contribute significantly to the total current consumption. Therefore the OCDEN fuse should be disabled if not needed.

Table 2. Fuse Bit Locations

Bit # ATmega8535 ATmega16

Fus

e H

igh

Byt

e

7 S8535C OCDEN

6 WDTON JTAGEN

5 SPIEN SPIEN

4 CKOPT CKOPT

3 EESAVE EESAVE

2 BOOTSZ1 BOOTSZ1

1 BOOTSZ0 BOOTSZ0

0 BOOTRST BOOTRST

Fus

e Lo

w B

yte

7 BODLEVEL BODLEVEL

6 BODEN BODEN

5 SUT1 SUT1

4 SUT0 SUT0

3 CKSEL3 CKSEL3

2 CKSEL2 CKSEL2

1 CKSEL1 CKSEL1

0 CKSEL0 CKSEL0

2 AVR088 2537C–AVR–01/04

Printed on recycled paper.

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