ATtiny85

The ATtiny85 is an Atmel 8-bit AVR microcontroller. The ATtiny falls under the tinyAVR series of AVR controllers and is more limited in terms of available memory, IO pins, and peripheral sets when compared to the megaAVR series microcontrollers found in many Arduinos. However, the ATtiny trades these limitations for lower power consumption and smaller form factor.

A single ATtiny85 IC costs around $3-4 CAD shipped from China which seems quite expensive when compared to something like the ESP8266. A 5 pack I bought cost about $14 CAD.

Getting started

Connecting the ATtiny for ISP

ATtiny25/ATtiny45/ATtiny85 pinout

In-system programming (ISP) is used for programming the ATtiny. This is also sometimes referred to as ICSP (In-circuit serial programming).

The ATtiny85 in the DIP/SOIC/SOP package has 8 pins. The pinouts are shown on the right. To program the ATtiny, we will be using ISP via pins 5, 6, and 7 (outlined in orange in the diagram). Any ISP programmer will do, but we will be using an Arudino running the ArduinoISP sketch which can be found from the examples menu.

Once the ArduinoISP sketch is programmed on the Arduino, connect the ATtiny to the Arduino based on the table below.

Arduino Pin	Function	Connect to ATtiny85 Pin
10	Reset	1
11	MOSI	5
12	MISO	6
13	SCK	7

Connect ground and power to the ATtiny via the Arudino. You may use either 5 or 3.3 volts. A 10uF capacitor should also be connected between ground and the Arudino reset pin to prevent the Arduino from resetting during the ISP programming. At the end, it should look something like the following:

ATtiny85 ISP connection

Programming the ATtiny with AVRDUDE

AVRDude (AVR Downloader/UploaDEr) is a utility that can program AVR microcontrollers using ISP. Once the appropriate connections are made, we can use avrdude to read/write to the microcontroller.

avrdude takes some required options:

-c programmer-id - specify the programmer you are using. When using the Arduino as the ISP programmer, you may use the avrisp or arduino (which appears to be aliases for stk500) as the programmer.
-p part-number- the board you are targeting
-P port - the device or port that is connected to the ISP programmer. On Windows, it is probably some COM# port. On Linux, it's likely to be /dev/ttyUSB#.
-B bit-clock - Used to adjust the SPI speed. Default is 10, for 100KHz. KHz = 1000/(1.5+B).

Some useful commands to be aware of:

Task	Command
Read fuse settings	`avrdude -c avrisp -b 19200 -p attiny85 -P /dev/ttyUSB0 -U lfuse:r:-:i`
Change fuse settings	`avrdude -c avrisp -b 19200 -p attiny85 -P /dev/ttyUSB0 -U lfuse:w:0xXX:m -U hfuse:w:0xXX:m -U efuse:w:0xXX:m` Be careful when setting the fuses for an external clock. You may need to adjust the SPI speed in your programmer or via the -B option in avrdude.
Write flash	`avrdude -c avrisp -b 19200 -p attiny85 -P /dev/ttyUSB0 -U flash:w:firmware.hex`
Reading flash	`avrdude -c avrisp -b 19200 -p attiny85 -P /dev/ttyUSB0 -U flash:r:readout.hex:r`

Other notes

ATtiny configuration fuses

There are some configuration settings that can be set on the ATtiny. These settings can change the behavior of the microcontroller such as changing its clock speed or using an external clock source.

Despite its name, these fuses are really just flash/EEPROM values used as configuration parameters. You may change these values at a later time and are values don't actually blow any fuses.

There is a fuse calculator at https://www.engbedded.com/fusecalc/ for a wide array of AVR microcontrollers.

Comparison with other ATtiny microprocessors

The distinction between the other ATtiny microprocessors are:

	ATTiny25	ATTiny45	ATTiny85
Programmable Flash Memory (10k write/erase)	2K	4K	8K
EEPROM (100k write/erase)	128 bytes	256 bytes	512 bytes
SRAM	128 bytes	256 bytes	512 bytes
Program Counter bits wide	10	11	12

Tasks

Crazy Clock with ATTiny85

Implementing the crazy clock with an ATtiny85

The crazy clock project at https://github.com/nsayer/Crazy-Clock uses an ATtiny85 to change the behavior of clock movements driven by a lavet stepper motor. This is done by disconnecting the microcontroller in these clock movements and connecting the lavet stepper motor directly to the ATtiny85. This project includes some interesting clock behaviors such that ticks happen irregularly while still maintaining time overall.

To reduce power consumption, an external 32.768KHz clock is used with the ATtiny85. Two pins are used to drive the lavet stepper motor.

Quick guide

The bill of materials to get your own crazy clock are:

2x Schottky diodes
2x 100 Ohm resistors, 1x 10K resistor
2x ~15 pF capacitor
1x 32.768KHz crystal
1x ATtiny85
ISP programmer (an Arduino would do)
A random clock that uses a lavet stepper motor
breadboard and wires

Then,

Clone the crazy clock git repo. Install avrdude.
Wire up the ATtiny to the ISP programmer. Connect the crystal oscillator to XALT1/XALT2 with the 15pF capacitor on both pins tied to ground. Set the reset pin high using the 10K resistor.
Disconnect the lavet stepper motor from the existing microcontroller in the clock movement. Connect PB1 and PB2 to the lavet stepper motor via 100 Ohm resistors. A flyback diode should be added to ground.
Upload crazy clock to the ATtiny. This part is a little tricky since you need to change the fuses first, then adjust the programmer to use the slower SPI clock speed. First, set the fuses:
```
$ make fuse
```
After setting the fuses, make sure your programmer is using a speed of 250. If using ArduinoISP, change the line #define SPI_CLOCK (1000000/6) to #define SPI_CLOCK (1500/6). For some reason, -B 250 to avrdude doesn't seem to do the trick.
Then, upload the desired clock with make flash:
```
$ make flash TYPE=crazy
```
This step will take a while. If your clock is connected, the second hand should be going back and forth randomly, which is sort of interesting to look at. When avrdude tries to verify the flash by reading back the contents, it will fail. This seems to be OK.
Seed the randomness to the EEPROM.
```
$ make seed
```

If all goes well, the clock should begin ticking using the chosen clock type.

Issues

The lavet stepper motor seems to be most happy when the ATtiny is powered via ~4v rather than 5v. When using a higher voltage, the second hand twitches but doesn't have enough power to the next second. With a lower voltage such as ~3.3v, it doesn't have enough power to reliably tick as well.

Originally, the clock uses ~32ms pulses at 1.5v every second to advance the clock. Perhaps we need to ensure the current and voltage matches?

Clock pulses to lavet stepper

After much fiddling and probing, the problem disappeared after I cut the trace to the original clock IC again. One of the traces might not have been severed completely which was causing an uneven current flow through the lavet motor.

Resources

Schematics: https://hackaday.io/project/5880-crazy-clock#menu-files
Project page: https://github.com/nsayer/Crazy-Clock

Troubleshooting

programmer not responding

When using avrdude using an Arduino running the ArduinoISP sketch as the ISP programmer, I got:

avrdude: ser_recv(): programmer is not responding
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 1 of 10: not in sync: resp=0x00
avrdude: stk500_getsync() attempt 2 of 10: not in sync: resp=0xe0
avrdude: stk500_getsync() attempt 3 of 10: not in sync: resp=0xe0
avrdude: stk500_getsync() attempt 4 of 10: not in sync: resp=0xe0
avrdude: stk500_getsync() attempt 5 of 10: not in sync: resp=0x00
avrdude: stk500_getsync() attempt 6 of 10: not in sync: resp=0xe0
avrdude: stk500_getsync() attempt 7 of 10: not in sync: resp=0xe0
avrdude: stk500_getsync() attempt 8 of 10: not in sync: resp=0xe0
avrdude: stk500_getsync() attempt 9 of 10: not in sync: resp=0x00
avrdude: ser_recv(): programmer is not responding
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 10 of 10: not in sync: resp=0x00

You need to make sure that avrdude is talking to the Arduino at 19200 baud. When calling avrdude, pass in -b 19200.

flash failes due to invalid device signature

avrdude -c avrisp -p attiny85 -B 250 -P /dev/ttyUSB0 -b 19200 -U flash:w:crazy.hex

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.04s

avrdude: Device signature = 0x000000 (retrying)

Reading | ################################################## | 100% 0.05s

avrdude: Device signature = 0x000000 (retrying)

Reading | ################################################## | 100% 0.04s

avrdude: Device signature = 0x000000
avrdude: Yikes!  Invalid device signature.
         Double check connections and try again, or use -F to override
         this check.

This happened after changing the fuses to use an external clock. Change the SPI speed in the ArudinoISP sketch to use 1500/6 if using a 32KHz oscillator with no prescaling. Change the line #define SPI_CLOCK (1000000/6) to #define SPI_CLOCK (1500/6) and reupload the ArduinoISP sketch before trying again.