AVR Mega128

The manual of the Mega128 (Rev 2467J-AVR-12/03) is somewhat intransparent and distributes the relevant information over many pages.

the fuses

The programming fuses are

fuse	default	meaning	page
CKSEL3..1	000	select the clock from internal RC, ceramic, XTAL, external clock	35, 36
CKSEL0	1	select the clock from internal RC, ceramic, XTAL, external clock	35, 36
CKOPT	1	clock amplifier for crystal oscillator or so, depends on the clocksource itself	36
SUT1..0	10	startup time selector	37
OCDEN	1	never ship as programmed	290
SPIEN	0	enable SPI programming mode	290
JTAGEN	0	enable JTAG programming	290
EESave	1	preserve EEPROM through chip erase	290
BOOTSZ1..0	00	select boot sector size as 00=4k at F000, 01=2k at F800, 10=1k at FC00, 11=512 at FE00	290
BOOTRST	1	select boot reset vector, 0 = in bootsector, at 0000 otherwise, set IVSEL in MCUCR register for the interrupts	290
BODLEVEL	1	brownout detection on 2.7V = 1, or 4.0V = 0	290 & 50
BODEN	1	brownout detection enabled = 0	290 & 50

Concerning the fuses, 0 means programmed and 1 means unprogrammed.

default clock source

The device is shipped with CKSEL3..0 = 0001 and SUT1..0 = 10, an internal RC with the longest startup time. page 35.

crystal oscillator

This mode allows for either a crystal or a ceramic resonator. When CKOPT=0, then the Osc output at XTAL2 is being driven rail-rail. When not programmed (=1), then the swing is smaller, the frequency range is limited and the powerrconsumption is lower.
For resonators, the max frequency is 8MHz with CKOPT=1 and 16MHz with CKOPT=0.

Crystal oscillator operating modes, page 36

CKOPT	CKSEL3..1	freq range MHZ	recommended caps
1	101	0.4 - 0.9	for resonators only
1	110	0.9 - 3.0	12 - 22pF
1	111	3.0 - 8.0	12 - 22pF
0	101, 110, 111	1.0 -	12 - 22pF

Startup times for the crystal oscillator clock selection, page 37

CKSEL0	SUT1..0	startup time	additional reset delay	recommended usage
0	00	258 clk	4.1ms	ceramic, fast rising power, not frequency stable at startup
0	01	258 clk	65ms	ceramic, slow rising power, not frequency stable at startup
0	10	1k clk	-	ceramic, BOD enabled, frequency stable at startup
0	11	1k clk	4.1ms	ceramic, fast rising power, frequency stable at startup
1	00	1k clk	65ms	ceramic, slow rising power, frequency stable at startup
1	01	16k clk	-	crystal, BOD enabled
1	10	16k clk	4.1ms	crystal, fast rising power
1	11	16k clk	65ms	crystal, slow rising power

Low frequency crystal oscillator

This clock option is to use a 32768Hz crystal. Set the CKSEL=1001. Programming the CKOPT=0 enables internal 36pF between XTAL1 and GND and between XTAL2 and GND. Startup times for the low frequency crystal oscillator, page 37

SUT1..0	Startup from powerdown & powersave	additionaldelayfrom reset @ Vcc=5V	recommended usage
00	1k clk	4.1ms	fast rising power or BOD enabled
01	1k clk	65ms	slowly rising power
10	32k clk	65ms	Stable frequency at startup
11	reserved	reserved	reserved

option 00 and option 01 should only be used if frequency stability at startup is not important

External RC oscillator

The frequency is roughly 1/(3*R*C), with C at least 22pf between XTAL1 and GND.
The CKOPT fuse enables the internal 36pF cap between XTAL1 and GND.
Frequency selection, page 38

CKSEL3..0	Nominal frequency
0101	< 0.9 MHz
0110	0.9 - 3.0 MHz
0111	3.0 - 8.0 MHz
1000	8.0 - 12.0 MHz

Startup times for the external RC oscillator, page 38

SUT1..0	Startup from powerdown & powersave	additionaldelayfrom reset @ Vcc=5V	recommended usage
00	18clk	-	BOD enabled
01	18clk	4.1ms	fast rising power
10	18clk	65ms	Slowly rising power
11	6clk	4.1ms	fast rising power or BOD enabled

option 11 should not be selected close to the max frequnecy of the device

Calibrated internal RC oscillator

Set CKOPT=1
Frequency selection

CKSEL3..0	Nominal frequency
0001	1.0 MHz
0010	2.0 MHz
0011	4.0 MHz
0100	8.0 MHz

The 0001 is factory default.

Startup times for internal calibrated RC oscillator, page 39

SUT1..0	Startup from powerdown & powersave	additionaldelayfrom reset @ Vcc=5V	recommended usage
00	6clk	-	BOD enabled
01	6clk	4.1ms	fast rising power
10	6clk	65ms	Slowly rising power
11	reserved	reserved	reserved

option 10 is default when shipped.
The calibration value is written to register OSCCAL which is not available in Mega103 mode.

Internal RC Osc calibration value, page 40

OSCCAL value	min frequency in percentage from nominal	max frequency in percentage from nominal
$00	50	100
$7F	75	150
$FF	100	200

external clock

Connect the external oscliiator to XTAL1 while leaving the XTAL2 open.
Set Cksel3..0=0000, CKOPT is 1 or a 36pf cap is enabled between XTAL1 and GND.

Startup times for external clock, page 40

SUT1..0	Startup from powerdown & powersave	additionaldelayfrom reset @ Vcc=5V	recommended usage
00	6clk	-	BOD enabled
01	6clk	4.1ms	fast rising power
10	6clk	65ms	Slowly rising power
11	reserved	reserved	reserved

Questions ?
Suggestions?
Feedback ?

sponsored links

embedded page
home

last updated 27.may.05 or perhaps later

Copyright (99,2005) Ing.Büro R.Tschaggelar