June 10, 2021:
Revised: v1.0

ATtiny85 Fuse Restorer and Memory Erase Tool 

Have you 'bricked' your ATtiny85? Did you accidentally set the Lock fuses? Do you need to reset your ATtiny85 fuses before you can reprogram the flash and EEPROM memory? Here's a low-cost easy to build tool to restore the memory and fuses of an ATtiny13, ATtiny25, ATtiny45 or ATtiny85 back to factory default condition. It doesn't require a collection of power supplies or a special battery or an LCD. One pushbutton does the job! 


A number of my ATtiny85-based designs make use of all 8 pins on the chip. This requires the internal ‘fuses’ in the ATtiny85 to be programmed such that the function of Pin 1 changes from its factory-default "Reset" input mode into a semi-standard I/O pin mode.
Figure 1 : My prototype CEFR uses a single-sided PCB along
with a small handful of cheap widely available parts

However, if I need to change the software programmed in that chip at some later date, those fuses must first be changed in order to return Pin 1 to its original "Reset" mode. But that’s not possible because Pin 1 no longer has the necessary Reset functionality to allow it to be done! It’s a “Catch-22” situation.

For those unfamiliar with the term, a "Catch-22" situation is one where you are trapped by two contradictory conditions. For example, if you are looking for your first job and all the job adverts say you need work experience. But to get work experience, you need to have had a job! The term comes from the famous book of the same name written by Joseph Heller in 1961.

Reprogramming Options

Fortunately, there is a way around this problem! It requires a specialized High Voltage Serial Programmer (HVSP) or a less complex HVSP fuse resetting tool. These use a short pulse of +12V along with a specialized serial signaling protocol carried over four pins on the affected chip to restore those fuse bits.

I realized that builders of my SugarCube VFOs, for example, would probably require one of these to allow them to calibrate the Si5351a frequency or to reprogram the frequencies and settings in the EEPROM for the SC+4, SC+16 or SC+99 channel-based SugarCube designs.

These can be relatively expensive to buy. Since most users might only require such a device perhaps once every few years, or maybe only once, it’s very hard to justify such a purchase.

One way to solve this problem is to build a less complex HVSP fuse resetting device. 
These ‘fuse resetters’ are used to restore a 'bricked' chip which no longer responds to a low cost programmer like the USBasp due to incorrect fuse settings including incorrect clock settings. Most such DIY designs are reasonably cheap to make.

Existing DIY Fuse Resetter Designs

I thought I would simply create a link to an existing design and the job would be done. Unfortunately, as I searched for a suitable design, most of those I found seemed to have limited functionality or features that were completely unnecessary or were difficult to use.

For example, while many of these simple fuse reset tools could (usually) reset the “bricked” chip’s fuses back to the original factory default setting, many could not reset chips where the user had accidentally programmed the Memory Lock fuses. Lock fuses are used to prevent copying of software.

In order to restore those chips, it is first necessary to erase the chip’s flash memory. This can also result in the chip’s EEPROM being erased, by the way. Once that’s done, the fuses can finally be restored. Relatively few DIY fuse restorer designs published could do this yet, to my mind, that’s an essential requirement.

Most designs I encountered also required either a small 12VDC power supply, or a 9V battery, or a combination of 12V and 5V power supplies. Such arrangements are not always available or very convenient. It's fairly easy to mix up these two power supplies with unfortunate and generally destructive consequences.

A few designs also featured LCD displays. These prompt the user on each step required to restore the target AVR chip. They also display fuse settings before and after the process. Some designs featured additional jumper settings ehich allowed the user to restore fuses to specific non-factory-default settings.

All of these features are rarely required in a device that only ever really needs to do just one job – Restore the chip back to it’s factory-default condition with fuses reset and memory erased.

Finally, a further problem I routinely discovered was that the required software for these designs was often only available in the form of a C-language source file requiring compiling before it was ready to use. I'm not a fan of C. I don't read Swahili or ancient Cuniform either, both being orders of magnitude clearer and easier to understand.

Yes, it was a case of having to design and build one myself.


Here is my version of a Chip Eraser and Fuse Resetting tool (CEFR). It focuses on doing just that one task. It has a red LED to show when power is connected, a single pushbutton (‘START’) to do the job, and a green LED to indicate the result.

It runs from a standard USB port or 5V USB ‘wall-wart’ power supply.

That’s it. Nothing difficult, expensive or hard to find.

A low cost ATtiny2313 lies at the heart of the CEFR. It turns the various power supply rails to the target device on and off, handles the HVSP signaling protocol required with the target chip, and indicates the result to the user with a simple green LED. The LED turns on continuously (for about 20 seconds) to indicate success, or it blinks briefly to indicate failure.

Everything is operated from a USB 5V power supply. The PCB’s USB Type-B female connector allows, for example, a spare printer cable to be used to connect the CEFR to either a spare port on a nearby laptop or computer, or to a standard USB-type ‘wall wart’.

A 12V low current supply is required for the HVSP erase/fuse-reset process. Other fuse restorer designs use special boost converter chips such as the legacy MC34063, or parts like the MAX606, LT3464 or MT3608. However, such chips are not always easy to obtain. I never seem to have the right one in my parts bin. They can be costly, too.

To avoid that problem, this design uses some commonly available inexpensive discrete components to form a boost converter to generate 12V on the PCB.

A momentary START pushbutton controls the device, and a pair of LEDs provide indications to the user about power and operation. Press it briefly to erase the chip memory and restore the fuses. All quite simple.

Schematic Details

Figure 2 shows the schematic for the CEFR.

Figure 2 : Schematic of the Chip Eraser and Fuse Restorer

The arrival of 5V from the USB connector is indicated with the red LED (D4).

The majority of parts are used in the 5V to 12V boost converter. These are all the parts around Q2, Q3 and Q4. Q2 is the main boost converter switching device, turning on and off as a result of the feedback provided by C3, Q4 and resistors R6 and R7. L1, an inexpensive 100uH choke, boosts the incoming 5V rail via diode D2, a readily available 1N4148. The rectified output voltage is stored by C4.

The boosted voltage is detected using a small 400mW 12V Zener diode D3 which turns on and off Q2 via its base-emitter junction. Q2, in turn, controls the Q3/Q4 oscillator on/off ratio to keep the output of the boost converter close to 12V.

This little boost converter is not only surprisingly efficient, but it also achieves a very fast turn-on time to give +12V in less than 5mS after the 5V supply is applied.

The ATtiny2313 (IC1) switches this voltage supply on and off to the Reset pin (Pin 1) of the target ATtiny85 via Q1. IC1 also turns on and off the 5V supply to the target chip (IC2), and handles the signalling protocol for erasing and fuse reseting via four further I/O lines (SCI, SDO, SII and SDI). A detailed description of the protocol can be found in every AVR processor datasheet.

Finally, the result of the whole process is signaled to the user via the green LED (D1).


To simplify building the CEFR, a single-sided PCB measuring 85 x 35 mm (3.3” x 1.3”) has been designed for the unit. The overlay is shown in Figure 3. A smaller PCB is possible, but this single-sided PCB layout makes it easy to make the PCB at home. It also provides plenty of space for the various components making it easier to build.

Figure 3 : The CEFR can be built using this single-sided PCB layout
which is available for in the Downloads section

IC sockets are used for IC1 and IC2. If your CEFR is likely to be used frequently, then a “machine screw” IC socket is recommended for IC2. That's the IC socket for the chip to be erased and restored. The price difference between these higher quality 8-pin sockets and the least expensive types is minimal. Similarly, if you are making one of these for use
by an amateur radio club who are making a number of my SC+ VFOs, then fitting an 8-pin DIL ZIF (zero insertion force) socket may be an even better choice.

Build the boost converter section first and test it before going further. Once you've verified you have 12V (i.e. 11.5 - 12.5V)
on the test point (TP), then fit the sockets for IC1 and IC2, the two LEDs and related parts, and the Start switch.  Then complete the remainder of the PCB construction.

Note the two wire links must be fitted. They are just below and run parallel to IC1


The ATtiny2313 should be programmed using the HEX file available in the Download section below. This has been adapted from a variety of existing software from other designs including those of Peter Fleury, Dick Cappel, Uri Shaked and PE0FKO.

The fuse settings for the ATtiny2313 (IC1) are:

LOW: &h64    HIGH: &hDF     EXT’D: &hFF

These are also the default fuse settings for an ATtiny2313 (8MHz internal RC clock, Clock divided by 8) so it's quite possible you may not need to program those fuses at all.


  1. Insert the ATtiny85 to be erased and restored into the TARGET 8-pin socket (IC2)
  2. Connect USB power to J1 (Use a standard USB printer cable, for example, between J1 and a USB port on a computer or 5V USB wall-wart power supply)
  3. Check the POWER LED (Red) is on
  4. Press the START button briefly
  5. The STATUS (Green) LED should turn on and stay on (for about 20 seconds) to indicate success, or it will blink slowly to indicate a problem
  6. Disconnect the power from J1
  7. Remove the ATtiny85 from the Target socket. 
If the CEFR process was successful, the target ATtiny85 is now erased and ready for reprogramming with your usual programmer.


arrowSoftware: Click here to download the ZIP file containing the HEX file for programming of your ATtiny2313

arrowPCB Layout:  The Gerber files and JPG negative file are available here for the PCB.

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