Convenient, minimalist breakout/development board for any 24-pin tinyAVR 0/1/2-series (3227, 1627, 3217, 1607, etc) - bare board version

Over the past several years, Atmel/Microchip has been releasing parts based on their "avr8" architecture. These use the familiar AVR instruction set (with slightly improved timings) and open source avr-gcc compiler used with the classic AVR microcontrollers, but with redesigned and more capable on-chip peripherals and highly competitive prices. The best known of these (among hobbyists, at least) is the ATmega4809, used on the Arduino Uno WiFi Rev.2 and Nano Every. However, there is also an extensive line of ATtiny parts based on this new architecture as well as the flagship AVR devices in the Dx-series My megaTinyCore and DxCore packages provide full Arduino support for this exciting new architecture.

This is available as an assembled board with a 3227, 3217, or while supplies last, a 1607

Board dimensions are 1.9" x 0.85"

Special features of these breakout boards

The Rev. D has the latest version of the jumpers to make use of the unused pins on the 6-pin FTDI connector - CTS is connected to UPDI by default, so that you could combine the programmer and serial comms into one connector (we are developing a UPDI programmer that can switch between serial passthrough and UPDI programming that will be most convenient in this configuration) but CTS can be routed to PB0 (USART0 XDIR - for RS485) or ground (for applications that detect a connected device by it's pulling CTS low).

Likewise, DTR can be either routed to a pin (PB1). likely most useful for 0/1-series parts to use as an "ersatz reset pin" that would perform a software reset and be triggered by the falling edge - or it can be used with the usual Arduino RC autoreset circuit to generate a pulse on PA0 (can be reset on any 0/1/2-series part - but at the cost of losing the UPDI pin and needing to use either a bootloader or an HV programmer to reprogram; the latter requires disconnecting this solder bridge) or PB4, which on the 2-series only can be set to be the alternate reset pin, so you don't need to mess with HV UPDI programming.

Finally, all sources of power: the Vcc pin on serial, on the UPDI header, and the output of the regulator have a cuttable jumper if you need to disconnect it (for example if you're making something that runs directly from a LiPo battery, you would need to avoid connecting the battery straight to a 3.3v (or god help you a 5v) serial adapter's Vcc. The regulator output, serial Vcc pin, and UPDI programmer Vcc pin disconnect jumpers are marked V_REG (top right next to tab of regulator), V_SER, and VUP respectively.

For more information about using the modern tinyAVR 0/1/2-series with Arduino, see the megaTinyCore documentation, now more comprehensive than ever.

Soldering on the quad flat no lead (QFN) package that these parts are available in is not for a soldering novice. It can be done without as much difficulty as many people think, provided you have good eyesight and good equipment. It can be done with either iron or hot air using reasonably good 158C or 138C solder paste (the 138C stuff is also available in wire form, but is not common). Soldering with 183C tin/lead solder requires considerably more care, as it is far harder to correct any misalignment. Regardless of what method is used to solder the part all four sides should be examined afterwards, after removing any flux residue that might obstruct your view of the joint with 91-99% isopropanol. Bridges are most easily removed using a tinned-face-only bevel soldering iron tip or knife tip (apply a small amount of flux - preferably the clear type (like Icing SD360 or UGain UG78 as opposed to a more aggressive, rosin based flux that will darken and make it harder to see whether you were successful) to the bridge(s), wipe the tip quick;y on either a damp sponge or cotton cloth to remove any excess solder, and then melt the solder bridge and drag the iron away from the chip. That is surprisingly effective, and will usually clear the bridge unless there is either far too much solder (use desoldering braid) or the chip is misaligned such that one or more sides have the pins on the chip located in-between two pads. Fixing that requires all of the solder under the chip to be melted. With low temp solder this is not terribly difficult, but even with tin/lead solder, it requires good equipment to do with hot air, (and forget about using an iron or hot air on traditional lead free solder). A desoldering hotplate is highly effective, even a low quality one - though those often overheat the board, leading to the silkscreen discoloring.

We also sell a board in the same spirit for the 20-pin (ATtiny parts with part numbers ending in 06, 16, or 26) and 14 pin (part no. ending in 04, 14, or 24) 0/1/2-series tinyAVR parts, and for the 8-pin (part no. ending in 02 or 12) 0/1-series ones, and assembled boards are also available in those sizes as well (though we are no longer assembling 0-series breakout boards). All of the parts except the 24-pin ones are available in an SOIC package which is much easier to solder than a QFN.

ATtiny3217 features and Arduino support

• 32Kb flash, 2k SRAM (The 1617 has 16k flash and the same 2k SRAM while the 817 has just 512b of SRAM) that's the same as an ATmega328p!
• 21 available I/O pins
• 8 PWM pins (6 on TCA0, 2 on TCD0) with 8-bit resolution.
• Servo, Tone, Serial, SPI and Wire (I2C) support "just works". Can act as both master and slave at the same time (and this is exposed by the improved Wire library).
• Serial supports additional options like half duplex open drain mode, control of an external RS485 driver, and more.
• 2 Type B timers (1617, 3217 only) - so Servo and Tone can both be used simultaneously, or one or both can be taken over for input capture, periodic interrupt, or other functionality.
• DAC output (just do analogWrite() on the DAC pin - voltage is between 0V and the DAC reference voltage set in the tools menu; this voltage must be less than Vcc).
• 11 analog inputs, referenced to Vcc, external reference (1617, 3217 only), or one of the 5 built-in analog references - plus a whole second ADC on 1617 and 3217 parts which is not touched by the core.
• Internal clock at up to 20MHz at 5v, up to 10MHz at 3.3v.
• 3 Analog Comparators (1617, 3217 only - otherwise just 1)
• 2 configurable custom logic units.
• 2 synchronous and 4 asynchronous event channels - the event system is wierd on the 0/1-series, and overall inferior to the 2-series, though capable of the much the same things; the Event library included with megaTinyCore hides as much of his ugliness from you as possible.

ATtiny3227 differences

The newer ATTiny3227 adds some compelling improvements, but also yanks out a few features, so it's not strictly an upgrade, but is more often than not better. They're priced about the same. Extra 1k SRAM in 32k parts. Two more CCLs, and all 6 event channels work like they do on the Dx-series (ie, each one has sync and async subchannels, so you don't have to keep two kinds of generators and users straight) and the generator options are more uniformly distributed. The TCBs can count events. Oh and in place of the (dual) 10-bit single-ended ADC, you get a 12-bit differential ADC with a programmable gain amplifier (up to 16x, usable in single ended mode), with 15 single ended or positive channels and 7 negative channels. The signal can be sampled around twice as fast at the 1-series, or faster with an external reference, and can automatically accumulate up to 1024 readings for oversampling & decimation to achieve a theoretical 17bit accuracy. This is also a true differential ADC - in contrast to the Dx where the voltage had to be below VREF, on the 2-series, it can even go a fraction of a volt beyond the power rails, allowing you to, say, have power connected throug ha current sense resistor on the high side, and use the differential ADC to measure the voltage drop across that resistor using the smallest reference and largest gain (the lower the value of that sense resistor, the less it impacts the voltage the ATtiny sees, which is great for everything except sensing the current through the sense resistor)/ Only 6 PWM pins (no timer D) All parts have 2 TCBs and a second USART. * Dozens of errata that impacted the 0/1-series have been corrected.

ATtiny1607

The ATtiny1607 is the top-end 0-series part in the 24-pin package. The 0-series is intended for low cost applications, and cuts a few features for a meager discount. The 0-series discount is less than 10% vs the 1-series version, and around 15% vs the 2-series. Don't forget that 16k 1/2-series parts have 2k SRAM, but 16k 0-series parts have only 1k SRAM. For hobbyists and one-off projects, the 3217 or 3227 is almost certainly a better deal. We do not plan to assembly more boards based on the tinyAVR 0-series moving forward. Demand has been poor, which is, in hindsight, unsurprising since they are strictly worse than the 1-series The main differences are: 16Kb flash, 1k SRAM Only 6 PWM pins (no timer D) Only 1 type B timer (Servo and Tone can't be used at the same time) No DAC. No external analog reference Only one ADC, only one analog comparator, only 3 event channels. None of those 3 can carry RTC-PIT events.

Special board features

• Every possible source of power, while defaulting to being connected, has cuttable bridge to disconnect it (if you're trying to prevent power supplies from fighting): VUP (UPDI header), VSER (6-pin Serial header) and VREG (on top side of board, next to tab of regulator - can also be used to reduce power consumption in standby if regulator not used.
• UPDI header (though not the pin) has a 470 ohm resistor in series with it. You can disable this if your programmers have their own resistors, by bridging the RBP pads on the back.
• The DTR line can be connected in any of three ways:
• Directly to PB1 (PB1 will be pulled low while DTR is asserted)
• Via the standard autoreset circuit as a pulse on PA0, the reset/UPDI pin.
• As above, except to the alternate reset pin, PB4.
• CTS need not go unused either - bridgeable pads can connect it to:
• Ground (for devices that use that as a signal that something is connected to a serial port)
• PB0 directly. CTS is in theory an output. Your finer serial consoles will display the state of such control lines,
• UPDI (directly). A number of community members, myself included, have put effort towards a scheme to the CTS line for UPDI, and thus get both programming and debug with a single serial cable. The idea would be use a special "serial adapter" that would react to a certain pattern of modem control lines to switch into UPDI mode, operate as Serial UPDI, then go back to being a serial adapter. I'm not aware of any complete successes though.
• Breakaway mounting tab with 2 holes suitable for a M3 or 4-40 screw on 0.55" centers
• 0.6" spacing of the rows of pins on either side allows those to be used with round or "machined" pin header, pointing downward for use with solderless breadboard (the larger square pin header will degrade the reliability of breadboard) or can be used to fit into wide SOIC sockets.
• Markings on pins show Arduino numeric pin number, port and bit (eg, PA2) and any serial interfaces that can use this pin (TWI, USART, and SPI), as well as whether the pin might have PWM on it (we can't be sure before we mount the chip, because the 1-series has 2 extra PWM pins)
• Checkboxes on the back specify the configuration the board was shipped in (chip, regulator, and whether PA0 was anything other than UPDI).
• Depending on the circumstances at the time that a board is marked, they will be either marked with black industrial Sharpie brand marking pens, or UV curing solder mask material (when many boards are marked at once, we are more likely to use solder mask material, which we have found to be far less likely wear off - however, it is also harder to apply with the same measure of control as you have with a marker)
• Starting with Rev. D, there is a via in the middle of the exposed pad of the QFN, which helps applying heat from the bottom of the board to solder the EP
• Also starting with Rev. D, there is a set of pads for an external clock in 3225 package.

Differences between revisions

• Rev -. A. and B were made with yellow solder mask. Over time the board house has become rather disappointing with this color, and the text got hard to read.
• Rev. B was the first to introduce most of the solder-bridgeable pads/jumper, and narrowed spacing to 0.7".
• Rev. C added the CTS and DTR configuration bridgeable pads and switched to a much more readable, if more mundane, blue solder mask, added a button between PB4 (alt reset on 2-series and ground) and squeezed the sides of the board inwards to fit the 0.6" center to center spacing target, as well as improving routing and markings considerably. Unfortunately, it also left out the power connection to the chip. Oops. This was also when the most notable functions of many pins were added to the markings and when we moved the mounting holes to a breakaway tab.. Rev C was never shipped to customers.
• Rev. D Fixed that issu, and made the background under the checkboxes on the back silkscreen white instead of blue, which makes the markings show up far better. D1 (autoreset clamp diode) was switched from a SOD123 to the easier to handle SOT23 package. They make SOT23 diodes with only 1 diode inside, but they're about the same price and specs as ones with two diodes inside. That is what we use, because common anode and common cathode diodes in SOT-23 are commonly used and readily available. In order to fit everything onto the narrower board, we had to shrink the following capacitors from 1206 to 0805 C1 (decoupling) C2 (reg output and board-scale decoupling) and C6 (newly added for the optional and rarely used external clock). Oh and rev D uses ENIG (gold) surface treatment.

We are only assembling boards with the Rev. D now. We have considerable inventory of -, A, and B bare boards, and we are selling them at a major discount, including out usual buy-us-out-get-the-stencil-free deal!

Programming the tinyAVR 0/1/2-series parts

There are two approaches to programming these parts:

UPDI (recommended)

Unlike other AVR microcontrollers, these new parts are programmed via a "UPDI" single wire interface instead of ISP. While we previously recommended using a nano running jtag2updi to program these, now the SerialUPDI tool, included with megaTinyCore, allows programming of UPDI parts with just a standard serial adapter and a resistor or schottky diode. In light of the simplicity of the modification and low cost and abundance of serial adapters, SerialUPDI is our recommended programing tool.

The order of the pins on the UPDI header is UPDI-Gnd-Vcc - this means that in the most common configuration, a target board powered by the serial adapter's Vcc pin, the board will not suffer immediate harm if the UPDI programming connector is on backwards - the UPDI pin tolerates up to 12v for HV-override of UPDI pin function. The 470 ohm resistor on the breakout board is between the UPDI pin of the programming header and the actual I/O pin of the chip, so that the pin can be miswired without the current exceeding a non-damaging value, and that even in the event that the programmer and target were out of sync and trying to drive the pin in opposite directions, that would not exceed the maximum ratings of the part, even were the load to be continuous (which it likely would not be, since you'd notice that it wasn't working and likely quickly discover such a wiring error.

Over serial using a bootloader

Alternately, a serial bootloader can be used (megaTinyCore includes a compatible version of Optiboot and prebuilt binaries to support the typical usage scenarios) Because the Reset and UPDI functionality shares the same pin, on the 0/1-series parts it is only possible to have an external reset signal to begin an upload if UPDI has been disabled (requiring an HV UPDI programmer to restore). This is inconvenient, especially since not only are HV UPDI programmers uncommon, it is likely that with the DD and EA-series parts, a new tide of HV-UPDI-but-for-not-for-tinyAVR programmers will appear. Those parts have a dedicated pin which can be set to be reset or an input in addition to the UPDI pin which can be set to be a GPIO. The critical difference is that on those parts, the reset/input only pin gets the HV pulse to override the pin functions, instead of the UPDI pin. That is a Big Deal, and makes design of the programmer much easier - you don't need to worry about +12v damaging the programmer - the reset line and the output of a 5v->12v boost or capswitch dc-dc converter is all that is needed.

Optiboot and Autoreset

megaTinyCore now supports the Optiboot bootloader for serial uploads (though you still need a UPDI programmer to bootload the board). For 2-series parts, which support a setting which moves the reset function to PB4 and which have a PB4 to move it to (such as the 3227). These get bootloader and PB4 reset enabled as standard. Uploading using a non-optiboot board selected will remove the bootloader, and "Burn Bootloader" with a UPDI programmer can be used to change PB4 back into a normal pin.

On the 0 and 1-series parts, and the 14-pin 2-series, since there is no alternate reset option, the most common way to enter the bootloader is on POR (power on reset) - you would plug and unplug power from the board not more than 8 seconds before the IDE attempts to upload to it (similar to the old digispark boards). Timing this can be frustrating, and the user experience is generally not great. However the alternative to get autoreset requires that you configure the UPDI pin as reset. Reversing this requires an "HV UPDI programmer" and those are not commonly found in the hands of hobbyists.

After turning a reset pin option, to get autoreset, you must connect the appropriate solder bridge on the back of the board: Pulse to PB4 or PA0 for a 2-series (yeah, you can set UPDI to act as reset on a 2-series that could use alt reset, if you're desperate for 1 more pin) - this can be done by just adding a small glob of solder to the appropriate pair of pins. Before using HV UPDI to restore normal UPDI functionality to the UPDI pin, you must disconnect this solder bridge, and if you turning PB4 into a normal pin instead of reset, you should also clear that solder bridge.

BOM (Rev. D)

• U1: Any 24 pin tinyAVR 0/1/2-series part 807, 1607, 817, 1617, 3217, 427, 827, 1627, or 3227.
• U2: (optional) 1117-series regulator. We use and recommend the LDL1117 for most purposes, as they get astonishingly low dropout voltage, work with cheap ceramic caps, and are priced similarly to other 1117-alikes with vastly inferior performance. If U2 is omitted, a 2520 0 ohm resistor may be placed to bridge the tab, middle pin, and Vin pin, to connect the Vin header directly to the power rail (useful for running from batteries).
• R1: 470 ohm 1206 - required if device may be connected to a UPDI programmer without any series resistance (like a JTAG2UPDI device with the pin of the programmer tied directly to the programming connector), otherwise it may be omitted and the RBP jumper on the underside bridged.
• R2: (optional) 10k 1206 if using "Autoreset". This is recommended only on 2-series parts because of the new alt-reset-pin feature that doesn't require sacrificing UPDI programming for a reset pin.
• R3: (optional) 220 ohm to 4.7k 1206 - ballast resistor for LED D2. Select resistor value based on how bright you want the LED to be.
• C1: 0.1uf Ceramic Capacitor for decoupling. This must be installed to ensure stable and correct operation.
• C2: 4.7uF 0805 Ceramic Capacitor for board-level decoupling and regulator output filtering. Be sure to read the relevant section of the datasheet for the regulator you chose for U2 (if any). It is strongly recommended to put a 4.7uF cap here, even if no regulator is installed.
• C3: (Install only if U2 installed) 4.7uF 1206 Ceramic Capacitor or as dictated by the regulator datasheet. using the physically larger capacitor gives better real-world capacitance as the DC voltage across the capacitor increases (0805 is okay up only to 5v, while the voltage coming into Vin may be higher than that
• C4: (Install only if Autoreset used in pulse mode) 0.1uF Ceramic 1206.
• C5: (Install only if Y1 installed) Ceramic 0805 capacitor, typ. 0.01uF. Consult the datasheet for Y1 for further guidance. This may also be installed for supplemental decoupling of the power to the chip in order to reduce noise in analog measurements (most relevant on the 2-series)
• D1: (install only if Autoreset used in pulse mode) SOT23 dual schottky diode, common cathode type. A single-diode in SOT-23 with the cathode on the center pin may also be used, but as these are not any cheaper, you might as well buy the dual-diode version. This package proved equally economical and much easier to handle than the previously used SOD123 diode.
• D2: (Optional) LED 1206 in your preferred color. We used white on 5v boards, green on 3.3v boards, and red on no-regulator boards, as those colors can be readily distinguished at a glance, making it easier to manage inventory (the die on red/orange LEDs is a deep red or orange, while green and blue LEDs do not have a distinct color, and white LEDs are encapsulated in neon yellow plastic vs clear plastic of single-color LEDs. The red and green portions of the visible spectrum are generated through fluorescence of that neon-colored plastic, which conceals a royal blue (440nm typ) LED) Y1: (Optional. C5 required if installed): An external clock generator in 3.2 x 2.5mm 4-pad SMT package can be installed here along with decoupling capacitor C5 to provide a more accurate clock (external crystals are not supported by these parts). You should cut the bridge between hole marked PA3 and Y1 for best results if Y1 is installed, to reduce EMI and improve clock stability - otherwise leave it in place. Y1 should be oriented with the output pin closest to PA3. If Y1 is oriented in the opposite direction, it will be instantly burned out if power is applied, failing to a near short circuit. F1: 1206 self-resetting polyfuse. Maximum current dictated by your power source and application. We use 0.5A ones for our assembled boards. If the regulator U2 is to be used, something must be installed here; if you are certain that current is limited to a safe amount upstream this can be replaced by a 0 ohm resistor. If U2 is not installed and Vin and Vout aren't bridged with a 0 ohm resistor as described under U2, this component should be omitted as it is not connected to the circuit in that case. Note: All lithium batteries larger than a coin cell should be treated as if they were unprotected unless obtained from a reputable distributor in your jurisdiction - not from a market place site or shipped in from overseas. Watch out for implausibly high capacity 18650 batteries on marketplace websites (anything over 3500 mAh) - they're fake, and should be treated as if unprotected, and generally avoided entirely, because the actual capacity is rarely more than 500 mAh, and sometimes virtually nil.

Recommended Pins

In some of the pictures shown above, the boards are shown with square pin header. Pin header sets are available as an option; in this case the UPDI and FTDI headers are right angle type, and UPDI, FTDI, Vdd and Gnd are white, yellow, red, and green respectively. The color coding can be a real help, especially in terms of preventing accidental reverse polarity situations, which generally result in destruction of hardware. These colored pin headers also look fantastic on pictures posted to social media ;-) We sell optional pre-cut pin header sets (note that you can't cut them with normal wire cutters, because that squishes the plastic enough that (in these boards for example) the Vcc or Gnd and I/O pins wouldn't quite fit. The same goes for breaking them off. We use a special cutting tool to precisely slice the header so that it can be used without this difficulty.)

TinyAVR 2-series vs AVR DD-series

You may have heard about the AVR DD-series, an extension of the high end AVR line down to flash sizes as small as 16k and pincounts as low as 14. Several times I have been asked which is better. The answer that it depends on what you're doing with them. We are working on getting Arduino support for the DDs working in DxCore, and already have hardware designs ready to put on sale then.

• The DD's have MVIO (essentially a built-in level shifter). If you need his feature, you need the DD.
• The DD ADC does not have a PGA, and is not a proper differential ADC - the maximum input voltage is VREF.
• The DD has more pin mapping options for USART0, and SPI0.
• The DD PORTMUX options don't start to shine until 28-32 pin parts.
• The DD can run at the full clock speed at the minimum voltage, can be clocked higher, and overclocks insanely well.
• The DD can use an external crystal - but it takes two pins to do so, with hurts at low pincounts. It also has the type D timer, like the 1-series had. It's still got most of the same bugs (other than the portmux one), and its still the most complicated peripheral in any non-xmega AVR I am aware of, so it's not something that a great many people will be using, but it does give you some extra PWM pins, at a minimum.
• The DD shows less flexibility in the design regarding the pin mapping in genera. A great example of that is the CCL. AVRxxDD20 has 20 pins and 4 CCL logic blocks. So does the ATtiny 3226. On the the 3226, all pin inputs except LUT1 IN1 and LUT1 IN2, and the alternate output pin for LUT2 That's 10/12 inputs and 7/8 outputs. On the DD 20, you get.... all the LUT0 pins, IN1 and IN2 and an output pin for LUT1 (no alt output), an alternate output pin for LUT. And that's it - 5/12 input pins, and 4/8 outputs. Now as I've said many times the point of the CCL is not to act as trivial glue logic, but to use the internal inputs, many of which are incredibly powerful. it underlines a general philosophical difference between tinyAVR and full-size parts. The tinyAVRs are not designed to be scaled up to 64 pins, and the design puts more effort on making as many peripheral inputs and outputs available as it can. On the Dx-series, consistency within the series and a coherent logical plan behind th3e pin assignment is instead held to be more important.

But really it's the first two bullet points that are most likely to be the deciding factor: If analog readings are a big part of your project, the 12-bit real differential ADC with PGA and 1024 sample accumulation beats the stuffing out of the 12-bit kinda-differential ADC with slower conversion (lower maximum ADC clock speed) with no PGA and 128 sample accumulation that the DD has. On the other hand, I know a lot of people were on the edge of their seat waiting for the low-pincount MVIO DD-series. So it's really all dictated by your application, and neither one is strictly better - in almost every design I've sketched out, only a DD or only a 2-series would work well. . Just don't forget that the swapped the position of power and ground on the DD's in the 20 and 14-pincount packages compared to the tinyAVR (like almost all ICs, applying reversed polarity to the power pins will near-instantly destroy the chip)

To clarify the volume discount: For Rev B and D boards there is a 25cent/board discount if you buy at least a minipanel (8). Because of the strange way Tindie makes us specify the price of product options, it makes less sense than it ought to.