ATtiny1634

Specification	ATtiny1634	ATtiny1634	ATtiny1634
Bootloader (if any)	None	Optiboot	Micronucleus
Uploading uses	ISP/SPI pins	Serial Adapter	USB (directly)
Flash available	16384 bytes	15744 bytes	TBD bytes
RAM	1024 bytes	128 bytes	128 bytes
EEPROM	256 bytes	64 bytes	64 bytes
GPIO Pins	17 + RESET	17 + RESET	17 + RESET
ADC Channels	12	None	None
PWM Channels	4	4	4
Interfaces	USI, 2xUSART	USI, USART	USI, USART
Interfaces	I2C slave	USI, USART	USI, USART
Clocking Options	in MHz	in MHz	in MHz
Int. Oscillator	8, 4, 2, 1	8, 4, 2, 1	8, 4, 2, 1
Int. ULP Oscillator	128 kHz	128 kHz	128 kHz
Internal, with tuning	8, 12, 12.8	8, 12, 12.8	8, 12, 12.8
External Crystal	Up to 16 MHz	Up to 16 MHz	Up to 16 MHz
External Clock	Up to 16 MHz	Up to 16 MHz	Up to 16 MHz
LED_BUILTIN	PIN_PC0	PIN_PC0	PIN_PC2

Overview

Unlike most classic ATtiny parts, the 1634 is only spec'ed for maximum clock speed of 12 MHz. However experience has shown that it generally works at 16 MHz @ 5V. The ATtiny1634R has a more tightly factory calibrated internal oscillator. It is otherwise identical, has the same signature, and is interchangible.

This is one of the last three classic tinyAVRs to be released, and you can see that they were playing around with things a bit, not always successfully.

Warning: PB3 does not work as an input unless watchdog timer is running

This is a design flaw in the chip, as noted in the datasheet errata. Additionally, when the "ULP" oscillator (used by the WDT, among other things) is not running, it is "internally pulled down"; phrased more pessimistically, one might say that "if pin is output and high, it will continually draw current even without an external load. Definitely don't try to use power-saving sleep mode with PB3 set OUTPUT and HIGH. See code for "workaround" below - but the pin is still less useful than it should be; it is best limited to active output while the chip is awake (such as via it's PWM capability). Unless of course you want to use the ULP/WDT....

Programming

The ATtiny1634 can be programmed by use of any ISP programmer. ATTinyCore 2.0.0 supports the following programming options:

Optiboot Bootloader

This core includes an Optiboot bootloader for the ATtiny1634, operating on either of the hardware serial ports, 0 or 1, at the standard ATTinyCore baud rates (which have changed in 2.0.0 for improved reliability see the Optboot reference. The bootloader uses 640b of space, leaving 15744b available for user code. In order to work on these parts, which do not have hardware bootloader support (hence no BOOTRST functionality), "Virtual Boot" is used. This works around this limitation by rewriting the vector table of the sketch as it's uploaded - the reset vector gets pointed at the start of the bootloader, while the EE_RDY vector gets pointed to the start of the application (versions of the core prior to 1.2.0 used WDT vector, so WDT cannot be used as an interrupt - we recommend burning bootloader with the new version if this is an issue). This bootloader is not production grade; it can fail to erase properly which requires ISP programming (reburning bootloader, nothing fancy - but often problematic in production situations)

Micronucleus Bootloader

New in 2.0.0. D+ is on PC4, D- is on PC5, and it tries to blink an LED on PC2. The Azduino USB 1634, an "ultramini" form factor designed to fit into a DIP socket, will be available from my Tindie store some time after the Chinese New Year holiday ends and PCBs can be manufactured again. It uses the internal oscillator tuned up to 12 MHz like the ATtiny84 and 841 do; it has not been tested thoroughly on this part, but at least the erase mechanism should be sound.

Features

Two pin mapping options

When I first merged in support for the tiny1634, I lacked the background to assess the pin mapping that the original author (TC-world as I recall) had chosen. With greater experience, it is evident that it is markedly worse than it could have been I think it was designed with it's prime directive to make 0 and 1 be the serial pins, which was very common practice in those days - and to try to have the SPI be on the same numbered pins as the '328p. As the market has become less of a monoculture, that convention has become less ubiquitous, which is good, because it's destructive . 2.0.0 introduces a more rational pin mapping with the pins numbered in clockwise order instead of counterclockwise. The desired pin mapping can be chosen from the Tools -> Pin Mapping submenu. Be very sure that you have selected the one that you wrote your sketch for, as debugging these issues can be surprisingly timeconsuming. Your sketch can check for PINMAPPING_CCW or PINMAPPING_CW macro (eg, #ifdef PINMAPPING_CCW - I would recommend checking for the incompatible one, and immediately #error'ing if you find it). Remember also that you can always refer to pins by their port and number within that port, using the PIN_Pxn syntax - where x is the port letter, and n is the pin number, eg PIN_PA7 is PIN A7, which is pin 7 in the clockwise mapping and pin 1 in the counterclockwise mapping. The clockwise mapping is strictly better than the counterclockwise one - the macros are more efficient (hence smaller binary sizes) Example of a "guard" against wrong pin mapping:

#ifdef PINMAPPING_CCW
  #error "Sketch was written for clockwise pin mapping!"
#endif

Internal Oscillator voltage dependence

Like the tiny828 and x41, While the calibration of the internal oscillator on the 1634 is very accurate between 2.7 and 4v, as the voltage rises above 4.5v, the speed increases significantly. Although the magnitude of this is larger than on many of the more common parts, the issue is not as severe as had long been thought - the impact is magnified by the fact that the bootloader used 57600 baud at 8 MHz - which is off by more than 2% even assuming a perfect oscillator due to baud rate math. The fact that many USB ports actually supply 5.2-5.3V to help charge cellphones of course only makes it that much worse. in 1.5.0 a workaround was implemented to try to make matters better on this front, but he new baud rate changes in 2.0.0 should provide substantial improvements in bootloader reliability here (see the Optiboot reference linked above).

We do still provide a >4.5v clock option in order to improve behavior of the running sketch - it will nudge the oscillator calibration down to move it closer to the nominal 8MHz clock speed; sketches uploaded with the higher voltage option. This is not perfect, but it is generally good enough to work with Serial on around 5v (including 5.25v often found on USB ports to facilitate chargeing powerhungry devices), and millis()/micros() will keep better time than in previous versions.

The internal oscillator is factory calibrated to +/- 10% or +/- 2% for the slightly more expensive 1634R. +/- 2% is good enough for serial communication. However, this spec is only valid below 4v - above 4v, the oscillator runs significantly faster; enough so that serial communication does not work absent the above-described countermeasures.

PWM frequency

TC0 is always run in Fast PWM mode: We use TC0 for millis, and phase correct mode can't be used on the millis timer - you need to read the count to get micros, but that doesn't tell you the time in phase correct mode because you don't know if it's upcounting or downcounting in phase correct mode.

F_CPU	F_PWMTC0	F_PWMTC1	Notes
1 MHz	1/8/256= 488 Hz	1/8/256= 488 Hz
2 MHz	2/8/256= 977 Hz	2/8/256= 977 Hz
<4 MHz	x/8/256= 488 * x Hz	x/8/512= 244 * x Hz	Phase correct TC1
4 MHz	4/8/256= 1960 Hz	4/8/512= 977 Hz	Phase correct TC1
<8 MHz	x/64/256= 61 * x Hz	x/8/512= 244 * x Hz	Between 4 and 8 MHz, the target range is elusive
8 MHz	8/64/256= 488 Hz	8/64/256= 488 Hz
>8 MHz	x/64/256= 61 * x Hz	x/64/256= 61 * x Hz
12 MHz	12/64/256= 735 Hz	12/64/256= 735 Hz
16 MHz	16/64/256= 977 Hz	16/64/256= 977 Hz
>16 MHz	x/64/256= 61 * x Hz	x/64/512= 31 * x Hz	Phase correct TC1
20 MHz	20/64/256= 1220 Hz	20/64/512= 610 Hz	Phase correct TC1

Phase correct PWM counts up to 255, turning the pin off as it passes the compare value, updates it's double-buffered registers at TOP, then it counts down to 0, flipping the pin back as is passes the compare value. This is considered preferable for motor control applications, though the "Phase and Frequency Correct" mode is better if the period is ever adjusted by a large amount at a time, because it updates the doublebuffered registers at BOTTOM, and thus produces a less problematic glitch in the duty cycle, but doesn't have any modes that don't require setting ICR1 too.

For more information see the Changing PWM Frequency reference.

Tone Support

Tone() uses Timer1. For best results, use pin 2 or 14 (PIN_PA6, PIN_PB3), as this will use the hardware output compare to generate the square wave instead of using interrupts. Any use of tone() will disable PWM on pins PA6 (Arduino pin 2) and PB3 (Arduino pin 14).

Servo Support

The standard Servo library is hardcoded to work on specific parts only, we include a builtin Servo library that supports the Tiny1634 series. As always, while a software serial port is receiving or transmitting, the servo signal will glitch. See the Servo/Servo_ATTinyCore library. Tone and Servo both require the same hardware resources are cannot be used at the same time.

I2C Support

There is no hardware I2C peripheral. I2C functionality can be achieved with the hardware USI. This is handled transparently via the special version of the Wire library included with this core. You must have external pullup resistors installed in order for I2C functionality to work at all. We only support use of the builtin universal Wire.h library. If you try to use other libraries and encounter issues, please contact the author or maintainer of that library - there are too many of these poorly written libraries for us to provide technical support for.

SPI Support

There is no hardware SPI peripheral. SPI functionality can be achieved with the hardware USI. This should be handled transparently via the SPI library. Take care to note that the USI does not have MISO/MOSI, it has DI/DO; when operating in master mode, DI is MISO, and DO is MOSI. When operating in slave mode, DI is MOSI and DO is MISO. The #defines for MISO and MOSI assume master mode (as this is much more common, and the only mode that the SPI library has ever supported). As with I2C, we only support SPI through the included universal SPI library, not through any other libraries that may exist, and can provide no support for third party SPI libraries.

UART (Serial) Support

There are two hardware serial ports, Serial and Serial1. It works the same as Serial on any normal Arduino - it is not a software implementation.

To use only TX or only RX channel, after Serial.begin(), one of the following commands will disable the TX or RX channels (for Serial1, use UCSR1B instead)

UCSR0B &= ~(1<<TXEN0); // disable TX
UCSR0B &= ~(1<<RXEN0); // disable RX

ADC Reference options

Note that when using the Internal 1.1v reference, you must not apply an external voltage to AREF pin - this sometimes appears to work, but other times results in erroneous ADC readings. Unlike some parts, there is no option to use the internal reference without the AREF pin being connected to it!

• DEFAULT: Vcc
• EXTERNAL: External voltage applied to AREF pin
• INTERNAL1V1: Internal 1.1v reference, AREF may have added capacitor for improved ADC stability.
• INTERNAL: synonym for INTERNAL1V1

Notice that the ADC is quite basic, I suspect that they had hoped to use something like what the 841 got, but were not able to have it ready in time to meet deadline.

Overclocking

Experience has shown that the ATtiny1634, operating at 5v and room temperature, will typically function at 16 MHz at 5v and room temperature without issue, although this is outside of the manufacturer's specification.

Purchasing ATtiny1634 Boards

I (Spence Konde / Dr. Azzy) sell ATtiny1634 boards through my Tindie store - your purchases support the continued development of this core.

• Assembled Boards
• Bare Boards
• Azduino 1634 Ultramini - Coming Q2 2022- fits in a DIP-24 socket (2 pins missing on each side to leave room for the chip).
• Azduino USB 1634 Ultramini - Coming Q2 2022 - fits in a DIP-24 socket (2 pins missing on each side to leave room for the chip), with a MicroUSB socket extending past one end.

PB3 silicon errata workaround

If you have no need to use the WDT, but do have a need to use PB3 as an input, you can keep the WDT running by putting it into interrupt mode, with an empty interrupt, at the cost of just 10b of flash, an ISR that executes in 11 clock cycles every 8 seconds, and an extra 1-4uA of power consumption (negligible compared to what the chip consumes when not sleeping, and you'll turn it off while sleeping anyway - see below) - so the real impact of this issue is in fact very low, assuming you know about it and don't waste hours or days trying to figure out what is going on.

// put these lines in setup
CCP = 0xD8; //write key to configuration change protection register
WDTCSR = (1 << WDP3) | (1 << WDP0) | (1 << WDIE); //enable WDT interrupt with longest prescale option (8 seconds)
// put this empty WDT ISR outside of all functions
EMPTY_INTERRUPT(WDT_vect) // empty ISR to work around bug with PB3. EMPTY_INTERRUPT uses 26 bytes less than ISR(WDT_vect){;}

If you are using sleep modes, you also need to turn the WDT off while sleeping (both because the interrupts would wake it, and because the WDT is consuming power, and presumably that's what you're trying to avoid by sleeping). Doing so as shown below only uses an extra 12-16 bytes if you call it from a single place, 20 if called from two places, and 2 bytes when you call it thereafter, compared to calling sleep_cpu() directly in those places, as you would on a part that didn't need this workaround.

void startSleep() { //call instead of sleep_cpu()
  CCP = 0xD8; //write key to configuration change protection register
  WDTCSR = 0; //disable WDT interrupt
  sleep_cpu();
  CCP = 0xD8; // write key to configuration change protection register
  WDTCSR = (1 << WDP3) | (1 << WDP0) | (1 << WDIE); // enable WDT interrupt
}

Interrupt Vectors

This table lists all of the interrupt vectors available on the ATtiny 1634, as well as the name you refer to them as when using the ISR() macro. Be aware that a non-existent vector is just a "warning" not an "error" (for example, if you misspell a vector name) - however, when that interrupt is triggered, the device will (at best) immediately reset (and not cleanly - I refer to this as a "dirty reset") The catastrophic nature of the failure often makes debugging challenging.

Note: The shown addresses below are "byte addressed" as that has proven more readily recognizable. The vector number is the number you are shown in the event of a duplicate vector error, as well as the interrupt priority (lower number = higher priority), if, for example, several interrupt flags are set while interrupts are disabled, the lowest numbered one would run first. Notice that INT0 is (as always) the highest priority interrupt. Since these parts use 4-byte vectors.

vect_num	Addr.	Vector Name	Interrupt Definition
0	0x0000	RESET_vect	Not an interrupt - this is a jump to the start of your code.
1	0x0004	INT0_vect	External Interrupt Request 0
2	0x0008	PCINT0_vect	Pin Change Interrupt 0 (PORT A)
3	0x0006	PCINT1_vect	Pin Change Interrupt 1 (PORT B)
4	0x0000	PCINT2_vect	Pin Change Interrupt 2 (PORT C)
5	0x0004	WDT_vect	Watchdog Time-out (Interrupt Mode)
6	0x0008	TIM1_CAPT_vect	Timer/Counter1 Input Capture
6	0x0008	TIMER1_CAPT_vect	Alias - provided by ATTinyCore
7	0x000C	TIM1_COMPA_vect	Timer/Counter1 Compare Match A
7	0x000C	TIMER1_COMPA_vect	Alias - provided by ATTinyCore
8	0x0010	TIM1_COMPB_vect	Timer/Counter1 Compare Match B
8	0x0010	TIMER1_COMPB_vect	Alias - provided by ATTinyCore
9	0x0014	TIM1_OVF_vect	Timer/Counter1 Overflow
9	0x0014	TIMER1_OVF_vect	Alias - provided by ATTinyCore
10	0x0018	TIM0_COMPA_vect	Timer/Counter0 Compare Match A
10	0x0018	TIMER0_COMPA_vect	Alias - provided by ATTinyCore
11	0x0016	TIM0_COMPB_vect	Timer/Counter0 Compare Match B
11	0x0016	TIMER0_COMPB_vect	Alias - provided by ATTinyCore
12	0x0010	TIM0_OVF_vect	Timer/Counter0 Overflow
12	0x0010	TIMER0_OVF_vect	Alias - provided by ATTinyCore
13	0x0014	ANA_COMP_vect	Analog Comparator
14	0x0018	ADC_READY_vect	ADC Conversion Complete
15	0x001C	USART0_RXS_vect	USART0 Rx Start
16	0x0020	USART0_RXC_vect	USART0 Rx Complete
17	0x0024	USART0_DRE_vect	USART0 Data Register Empty
18	0x0028	USART0_TXC_vect	USART0 Tx Complete
19	0x0026	USART1_RXS_vect	USART1 Rx Start
20	0x0020	USART1_RXC_vect	USART1 Rx Complete
21	0x0024	USART1_DRE_vect	USART1 Data Register Empty
22	0x0028	USART1_TXC_vect	USART1 Tx Complete
23	0x002C	USI_STR_vect	USI START
24	0x0030	USI_OVF_vect	USI Overflow
25	0x0034	TWI_vect	Two-Wire Interface
26	0x0038	EE_RDY_vect	EEPROM Ready
27	0x0036	QTRIP_vect	QTouch