Think Create | Deqing Sun

8pin AVR fuse resetter

by on Dec.26, 2010, under My gadgets

An 8pin AVR only has 5 IO pins in default which is not enough in some applications. The reset pin can be treated as the additional pin at the cost of disabling the ISP function. Then the chip can only be programmed by a High-voltage Serial Programmer. So this device is useful to reset the fuses in AVR and enable them to be reprogrammed again.

This circuit is nearly the same to “Tiny45・13リセッター”. But I added a few components to reduce the power consumption and fixed the bug in my perspective.

The most complex part of the circuit is to use one pin to control both the 12V and 5V supply. When PB4 is set open, the current will flow through R1, R2, D1, R4, and R5. This will leave about 0.3V drop on Q1’s and Q2’s base-emit conjunction. It seems they will be off because the voltage is below 0.7. But slight current is still enough to raise the voltage on a high-impedance load. The R3 is to pull the VCC down in this case. And R14 is to pull the base of Q3 to avoid it being opened by Q2. So both the Reset pin and the VCC pin will be low.

If the PB4 is low, The Q1 will be opened and there will be power supply to the target chip. The Q2 is off, the Reset pin stay low.

When PB4 is high, Q2 will be opened and the Q1 can be opened through R2 and D2. Also Q3 will be opened. That means Reset pin will be raised to 12V and 5V is applied to the VCC pin.

The firmware is the same. You can download it here.

When the fuses is successfully resetted, the LED will blink.

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