Atmel ATtiny 26, 261, 461, 861 Microcontroller Application Note

AVR504: Migrating from ATtiny26 to
ATtiny261/461/861
Features
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General Porting Considerations
Memories
System Clock and Clock Options
System Control and Reset
Registers
Interrupts
Timer/Counters
USI – Universal Serial Interface
Analogue-to-Digital Converter
I/O Ports
Memory Programming
Electrical Characteristics
8-bit
Microcontrollers
Application Note
1 Introduction
This application note is a guide to assist users of ATtiny26 in converting existing
designs to ATtiny261. The document will also assist ATtiny26 users to migrate to
the ATtiny461 and ATtiny861 devices, which are members of the same family as
the ATtiny261, offering larger memories.
In addition to the differences described in this document, the electrical
characteristics of the devices are different. Some of these differences are outlined
in this document and some are not. Please check the latest data sheets for detailed
information.
Improvements or added features in ATtiny261 that are not in conflict with those in
ATtiny26 are not listed in this document.
Rev. 8026B-AVR-04/08
2 General Porting Considerations
Between the devices described in this application note, some registers and register
bits have changed name but note that they preserve the same functionality. They are
all listed later in this document.
To make the porting process as easy as possible, always refer to registers and bit
positions using their defined names. Avoid using absolute addresses and values. In
most cases, the register and bit names are unchanged from device to device. When
you are porting a design, it is more convenient to include the correct definition file for
the new device, rather than manually changing all your addresses and bit values. It is
also considered good programming practice to use named references instead of
absolute values. Some examples are shown below.
PORTB |= (1<<PORTB3);
DDRB &= ~(1<<PORTB3);
// Set pin 3 on port B high
// Set pin 3 on port B as input
// Configure USI
USICR = (1<<USISIE)|(0<<USIOIE)|(1<<USIWM1)|(0<<USIWM0)|
(1<<USICS1)|(0<<USICS0)|(0<<USICLK)|(0<<USITC);
To avoid conflicts with added features and register functionality, never access
registers that are marked as reserved. Reserved bits should always be written to zero
if accessed. This ensures forward compatibility, and added features will stay in their
default states when unused.
3 Memories
ATtiny26 and ATtiny261 have the same amount of volatile and non-volatile memory.
ATtiny461 and ATtiny861 have more SRAM, Flash and EEPROM than ATtiny26.
Applications that rely on the size of memory may therefore misbehave when memory
size is increased. For example, this may be the case with wrap-around indexing of
EEPROM.
3.1 Stack
The ATtiny26 stack pointer is implemented as an 8-bit register in the I/O space. In
ATtiny261 the stack pointer is also realised as one 8-bit register, but in ATtiny461 and
ATtiny861 the amount of data memory exceeds the 8-bit address range and the stack
pointer has been realised as two 8-bit registers. See table below.
Table 3-1. Device Stack Pointers.
Device
Low Stack Pointer
ATtiny26
SP
ATtiny261
ATtiny461
ATtiny861
SPL
High Stack Pointer
N/a
SPH
See ATtiny261/461/861 data sheet for information on how to access stack pointer
registers.
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3.2 EEPROM
EEPROM write access times depend on the frequency of the internal RC oscillator. In
ATtiny261/461/861 the access times are shorter than in ATtiny26.
ATtiny261 is part of a pin and functionally compatible subfamily of tinyAVR®, where
the size of EEPROM ranges from 128 to 512 bytes. This means more than eight data
bits are required for memory addressing and, therefore, the EEPROM address
register has been expanded from one 8-bit register (EEAR in ATtiny26) to two
(EEARL and EEARH in ATtiny261, ATtiny461 and ATtiny861). Since the initial values
of the registers are undefined it is important to always write both registers even when
accessing only the bottom section of the EEPROM.
4 System Clock and Clock Options
ATtiny261/461/861 has a more advanced clock system than ATtiny26. In
ATtiny261/461/861 there is a system clock prescaler and two internal clock sources.
4.1 Clock Sources
Clock source options are set differently in ATtiny261/461/861. See table below.
Table 4-1. Clock Source Settings.
ATtiny26
ATtiny261/461/861
Clocking Option
PLLCK
CKSEL[3:0]
CKSEL[3:0]
CLKPS[3…0]
PLL
0
0001
0001
0000
External Clock
1
0000
0000
0000
Internal RC Oscillator, 1 MHz
1
0001
Internal RC Oscillator, 2 MHz
1
0010
Internal RC Oscillator, 4 MHz
1
0011
Internal RC Oscillator, 8 MHz
1
0100
External RC Oscillator, 0.1…0.9 MHz
1
0101
External RC Oscillator, 0.9…3.0 MHz
1
0110
External RC Oscillator, 3.0…8.0 MHz
1
0111
External RC Oscillator, 8.0…12.0 MHz
1
1000
External Low-Frequency Oscillator, 32.768 kHz
1
1001
01xx
0000
1
1010
0000
1
1011
1000
1001
1
1100
1101
1010
1011
0000
1
1
1110
0000
1
1111
1100, 1101
1110, 1111
External Resonator Oscillator, 0.4…0.9 MHz
External Crystal/Resonator Oscillator, 0.9…3.0 MHz
External Crystal/Resonator Oscillator, 3.0…16.0 MHz
0011
0010
0010
0001
0000
Not available, use internal RC
oscillator instead
0000
0000
0000
4.2 Calibration of Internal RC Oscillator
The frequency of the internal RC oscillator of ATtiny261/461/861 can be calibrated
using the OSCCAL register, similarly as in ATtiny26. The only difference is that in
ATtiny261/461/861 the highest bit (CAL7) determines the range of operation, while in
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ATtiny26 all eight bits of the OSCCAL register are used to adjust the frequency within
one, single range.
4.3 High Frequency PLL Clock
When using the internal PLL as a clock source, ATtiny261/461/861 applies a longer
start-up delay than ATtiny26 after a power-on reset and power-down wakeup. This is
to ensure stable and glitch-free operation.
5 System Control and Reset
ATtiny261/461/861 has more fuse bits than ATtiny26. In addition, some fuse bits have
a different functionality.
5.1 Brown-Out Detection (BOD)
Both ATtiny26 and ATtiny261/461/861 have programmable Brown-Out Detection
levels. The programming method and levels are not entirely the same. See table
below.
Table 5-1. Brown-Out Detection (BOD) Characteristics.
ATtiny26
ATtiny261/461/861
BODEN
BODLEVEL
VBOT
BODLEVEL[2…0]
VBOT
1
X
Disabled
111
Disabled
0
4.0 V
100
4.3 V
1
2.7 V
101
2.7 V
0
5.2 Start-up Times & Brown-out Detection
Due to electrical differences between ATtiny26 and ATtiny261/461/861 there may be
minor dissimilarities in start-up times. Please see device data sheets for more
detailed information.
5.3 Power-On Reset
The threshold levels of power-on reset are not identical for ATtiny26 and
ATtiny261/461/861. The power-on threshold voltage is slightly lower on
ATtiny261/461/861.
Please see device data sheets for more detailed information.
5.4 Watchdog Timer
The ATtiny261/461/861 includes an enhanced Watchdog Timer (WDT), compared to
the watchdog timer used in ATtiny26. At 5V operating voltage the WDT will behave
similar on ATtiny26 and ATtiny261/461/861. At lower voltages the watchdog timeout
interval may differ. Please see data sheet for characteristic data on watchdog
oscillator frequency.
If the WDT is not used, it is still recommended to disable it initially in the application
code to clear unintentional WDT enabled events.
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6 Registers
Some of the register names have changed and some registers have moved.
6.1 Renamed Registers
The below tables list the registers which have been renamed but still exist at the
same physical address and have maintained their functionality. It is only required to
update the register name in the application.
Table 6-1. Changes to Register Names.
Address [hex]
Name in ATtiny26
Name in ATtiny261/461/861
0x06
ADCSR
ADCSRA
0x08
ACSR
ACSRA
0x1E
EEAR
EEARL
0x1F
-
EEARH
0x32
TCNT0
TCNT0L
0x33
TCCR0
TCCR0B
0x3D
SP
SPL
6.2 Renamed Bits
The below table lists the bits that have been renamed, but still exist in the same
register and in the same register location.
Table 6-2. Changes to Bit Names.
ATtiny26
ATtiny261/461/861
Bit Name
Register
Address
Bit Name
ADFR
ADCSR
0x06
ADATE
USISIF
USISR
0x0E
USICIF
EEMWE
EECR
0x1C
EEMPE
EEWE
EECR
0x1C
EEPE
6.3 Removed Bits
The below table lists register bits, which have been removed and have no close
replacements.
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Table 6-3. ATtiny26 Removed Register Bits and Functions.
Register Name
Register Bit
Function
TCCR1B
CTC1
Resets Timer/Counter1
7 Interrupts
Interrupt handling has been improved in ATtiny261/461/861.
7.1 Interrupt Vectors
ATtiny261/461/861 has more interrupt vectors than ATtiny26, but all ATtiny26 vectors
exist in identical locations on ATtiny261/461/861. Programs can still use the end of
ATtiny26 interrupt vector table as a starting address on ATtiny261/461/861, provided
that ATtiny261/461/861 specific interrupts are not enabled.
7.2 Pin Change Interrupt
Pin change interrupts are automatically disabled on ATtiny26 if an alternative function
has been enabled for the given pin. In ATtiny261/461/861 this is not the case, where
alternative pin functions and pin change interrupts may be concurrently enabled for
the same pin. Pin change interrupts for a given pin in ATtiny261/461/861 are masked
and unmasked using Pin Change Mask Register A (PCMSKA) and Pin Change Mask
Register B (PCMSKB).
7.2.1 Source Pins and Masking
ATtiny261/461/861 extends pin change interrupts to cover all I/O pins, while ATtiny26
only includes pins PA3, PA6…7 and PB0…7. Also, ATtiny261/461/861 allows
individual masking of pin change interrupt sources via registers PCMSK0 and
PCMSK1 (a feature not found in ATtiny26) but the default setting is to enable only
those sources that are common for both ATtiny26 and ATtiny261/461/861.
8 Timer/Counters
Timer/Counter1 has been improved in ATtiny261/461/861.
8.1 Timer/Counter1
In ATtiny261/461/861 the synchronisation registers are not bypassed as in ATtiny26.
This means a synchronisation delay of 1.5 clock cycles is present in both
asynchronous and synchronous modes.
8.1.1 Bit CTC1 of Register TCCR1B
Bit CTC1 of register TCCR1B has been replaced by PWM1X on ATtiny261/461/861.
Please note that these bits have a different functionality and that this must be
considered during migration.
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8.1.2 Register OCR1C
Register OCR1C is now always the TOP value for the timer/counter and the register
is always reset (cleared to zero) after reaching the TOP value.
The lowest allowed TOP value is 3. If register OCR1C is loaded with a value lower
than 3 it will be forced to the value 3. Also note that after a reset the OCR1C register
is loaded with the timer/counter MAX value, which means that the default TOP value
is 0x3FF.
8.1.3 PWM Mode
When writing to register OCR1C on ATtiny261/461/861 the data is buffered and the
register updated only when OCR1C has reached the TOP value. In ATtiny26 writing
to OCR1C is immediate.
9 USI – Universal Serial Interface
The ATtiny261/461/861 uses an enhanced version of the Universal Serial Interface,
as compared to the ATtiny26.
9.1 Clock Sources
The USI clock can be selected from three different sources, two of which are common
for ATtiny26 and ATtiny261/461/861. The third clock source option (USICS[2:0] =
01X) in ATtiny26 is Timer/Counter0 Overflow, but in ATtiny261/461/861 it has been
replaced by Timer/Counter0 Compare Match. Similar functionality can be achieved
but it may be necessary to update any parts of firmware that refer to Timer/Counter0
overflow flags and interrupts.
10 Analogue-to-Digital Converter
The Analogue-to-Digital Converter (ADC) has been enhanced and in
ATtiny261/461/861 includes more features. ATtiny261/461/861 is a 1.8V device and
the default internal voltage reference is therefore set to 1.1V.
10.1 Voltage Reference Selection
Voltage reference source is selected using bits REFS2…0, located in registers
ADMUX (ADC Multiplexer Selection) and ADCSRB (ADC Control and Status Register
B). ATtiny26 only includes bits REFS1…0. The table below shows how to update
reference selection bits to maintain functionality.
Table 10-1. Setting Voltage Reference Source.
ATtiny26
ATtiny261/461/861
REFS1…0
REFS2…0
00
000
AVCC
01
001
AREF (internal reference turned off)
10
110
Internal 2.56V reference (AREF disconnected)
11
111
Internal 2.56V reference (AREF connected)
Voltage Reference
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11 I/O Ports
Test limits for port drive strength are lower on ATtiny261/461/861. This means
ATtiny261/461/861 ports are rated for a lower current than ATtiny26. Port driver
characteristics are outlined in the table below.
Table 11-1. Port Drive Characteristics.
Pin Rating [mA]
Sum Rating [mA]
ATtiny26
ATtiny261/461/861
ATtiny26
ATtiny261/461/861
VCC = 5V
20 mA
10 mA
VCC = 3V
10 mA
5 mA
400 mA
60 mA
Condition
12 Memory Programming
Some modifications regarding the programming of fuse and calibration bytes msut be
taken into account. Also, the functionality of the calibration byte has been changed.
12.1 Fuse Bits
The number of fuse bits has been increased and fuse bits are scattered over three
bytes in ATtiny261/461/861. Read and write algorithms must be updated for proper
fuse programming.
The functionality of the following fuse bits has changed:
• The BODLEVEL fuse bit of ATtiny26 has been expanded to three fuse bits
(BODLEVEL2…0) on ATtiny261/461/861.
• The BODEN fuse bit functionality of ATtiny26 has been integrated into
BODLEVEL2…0 fuse bits of ATtiny261/461/861.
• The PLLCK and CKOPT fuse bit functionality of ATtiny26 has been integrated into
CKSEL3…0 fuse bits of ATtiny261/461/861.
• Clock prescaler functionality of CKSEL fuses has been removed, since
ATtiny261/461/861 has a software configurable clock prescaler.
12.2 Signature Bytes
Signature bytes reside in a separate address space and can only be read external to
the device. Therefore, this notion only applies to programmers, et al, and not to the
actual program being migrated.
Signature bytes have been updated as illustrated in the table below.
Table 12-1. Summary of Signature Bytes.
8
Byte
ATtiny26
ATtiny261
ATtiny461
ATtiny861
$000
$1E
$1E
$1E
$1E
$001
$91
$91
$92
$93
$002
$09
$0C
$08
$0D
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12.3 Calibration Byte
ATtiny26 has four calibration bytes for the 1MHz, 2MHz, 4MHz and 8MHz operation
of the internal RC oscillator. ATtiny261/461/861 only has one calibration byte for the
internal RC oscillator.
13 Electrical Characteristics
ATtiny261/461/861 is manufactured using a different process than ATtiny26 and
electrical characteristics will therefore differ between these devices. Please consult
the data sheets for details on electrical characteristics.
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8026B-AVR-04/08