How to use reset pin as I/O pin with ATtiny44

Question

I am using the Arduino IDE as an ISP programmer to program my ATtiny44 IC. Unfortunately, I ran out of pins, so I want to use the reset pin as I/O. I read several articles online that says it is possible and even the datasheet says so.

I have a high voltage programmer as well. I wrote the code in the Arduino IDE and then used its hex file to program it using the high voltage programmer (TNM PROGRAMMER).

After I program, the IC stops working.

////////////////////////////////////EDIT/////////////////////////////////////

const int led = 0;
const int reset_led = 11; //physical pin 4 on attiny

// the setup function runs once when you press reset or power the board
void setup() {
    // initialize digital pin LED_BUILTIN as an output.
    pinMode(led, OUTPUT);
    pinMode(reset_led, OUTPUT);
}

// the loop function runs over and over again forever
void loop() {
    digitalWrite(reset_led, HIGH);  // turn the LED on (HIGH is the voltage level)
    digitalWrite (led, HIGH);
    delay(1000);                    // wait for a second
    digitalWrite(reset_led, LOW);   // turn the LED off by making the voltage LOW
    digitalWrite(led, LOW);         // turn the LED off by making the voltage LOW
    delay(1000);                    // wait for a second
}

New fuse bit setup:

////////////////////////EDIT 2 /////////////////////////////////////////

Now, the led on reset has turned off completely while the another still blinks. Powering my attiny44 with 5v.Code is same as before

////////////////////////////////EDIT 3////////////////////////////////////

#include <avr/io.h>
const int led = 0;
#define ledd PB3
// the setup function runs once when you press reset or power the board
void setup() {
  // initialize digital pin LED_BUILTIN as an output.
  pinMode(led, OUTPUT);
  
DDRB |= (uint8_t)(1<<PB3);
pinMode (ledd, OUTPUT);

}

// the loop function runs over and over again forever
void loop() {

PORTB |= (uint8_t)(1<< PB3);
 
 digitalWrite (led, HIGH);
  digitalWrite (ledd, HIGH);

 delay(1000);                       // wait for a second
 
PORTB &= ~((uint8_t)(1 << PB3)); // set PB3 to "low"
 
 digitalWrite(led, LOW);    // turn the LED off by making the voltage LOW
 delay(1000);                       // wait for a second
}

This code worked miraculously for me, Thanks to @thebusybee.

/////////////////////////////EDIT 3/////////////////////////////////////////

The PB3 pin is pin11 in arduino.

Answer 1

The data sheet gives insights:

Chapter 1.1.4 states:

The reset pin can also be used as a (weak) I/O pin.

"Weak" means here, that the output function cannot deliver as much current as the other outputs, according to figures 21-24 to 21-27 beginning at page 198 roughly 2 to 3 mA. This is commonly not enough for LEDs, except you use a high-efficiency LED.

And on page 66 is this relevant table (I cut the irrelevant column for PB2):

Table 10-8. Overriding Signals for Alternate Functions in PB[3:2]

Signal Name	PB3/RESET/dW/PCINT11
PUOE	RSTDISBL(1) + DEBUGWIRE_ENABLE(2)
PUOV	1
DDOE	RSTDISBL(1) + DEBUGWIRE_ENABLE(2)
DDOV	DEBUGWIRE_ENABLE(2) • debugWire Transmit
PVOE	RSTDISBL(1) + DEBUGWIRE_ENABLE(2)
PVOV	0
PTOE	0
DIEOE	RSTDISBL(1) + DEBUGWIRE_ENABLE(2) + PCINT11 • PCIE1
DIEOV	DEBUGWIRE_ENABLE(2) + (RSTDISBL(1) • PCINT11 • PCIE1)
DI	dW/PCINT11 Input
AIO

Note:

1. RSTDISBL is 1 when the Fuse is “0” (Programmed).
2. DebugWIRE is enabled when DWEN Fuse is programmed and Lock bits are unprogrammed.

Since you have DebugWIRE enabled, this seems to be the cause. The alternate function RESET is still active.

Please be aware that according to note 1 below table 19-4 on page 160:

After programming the RSTDISBL fuse, high-voltage serial programming must be used to change fuses and allow further programming.

This means that your circuit at PB3 needs to withstand the high voltage, and that the voltage source needs to provide enough current, which the circuit additionally draws at the high voltage.

---

Now to your code.

You use the pin number "0" and hope to have PB3. Well, according to this shamelessly linked pinout this is not correct:

Pin "0" is PA0.

PB3 has no number assigned.

Your next step could be to look up the sources of the Arduino library and check, if PB3 has a number assigned at all. I'm afraid this is not the case. If you're lucky and it has, use it. You will want to tell us about your finding then.

You can directly access the data direction control register and the port register. Grab your data sheet and other documentation, read, and implement.

This is untested:

// in setup():
    DDRB |= (uint8_t)(1U << 3);

// in loop():
    PORTB |= (uint8_t)(1U << 3);    // set PB3 to "high"
    PORTB &= ~((uint8_t)(1U << 3)); // set PB3 to "low"