Reset

Using Arduino General Electronics
1

Many common MCUs (ATtiny's ATmega's etc) have a reset pin for programming them. The datasheets always talk about the "minimum pulse length." What is this, and how can you use the reset pin as a reset pin AND as a GPIO w/out triggering a reset?

Thanks!
baum

2

In short, you can't.

From the datasheet: "If the RSTDISBL Fuse is programmed, PC6 is used as an I/O pin. Note that the electrical char- acteristics of PC6 differ from those of the other pins of Port C.

If the RSTDISBL Fuse is unprogrammed, PC6 is used as a Reset input. A low level on this pin for longer than the minimum pulse length will generate a Reset, even if the clock is not running. The minimum pulse length is given in Table 28-12 on page 323. Shorter pulses are not guaran- teed to generate a Reset."

The RSTDISBL (reset disable) fuse determines if the pin is a Reset or I/O.

3

So in my projects, I'm better off not using PC6 and just using the other 5?

4

Just tuned in to this thread. Thanks James. That was very clear. I'm taking notes. :slight_smile:

5

So in my projects, I'm better off not using PC6 and just using the other 5?

Well, "better off" if you want the ability to RESET the processor. If not, then it doesn't matter.

6

So then what is RESET?

7

I capitalized the word for emphasis. I assume from your first post you are familiar with the concept of resetting a microcontroller.

8

Reset is like the reset button on arduino (which is wired directly to reset pin). But why do I need to use reset w/ Arduino ISP? (which I will be using).

Thanks!

9

The ISP programs the microcontroller's on-board flash ROM. To make sure the processor is doing nothing while this occurs, the processor must be held in Reset.

10

But why do I need to use reset w/ Arduino ISP? (which I will be using).

You cannot program the chip using the ISP (ICSP in Arduinospeak) pins without the reset function enabled. If you want to use the reset pin for I/O then you have to program the chip via a bootloader.

Don

11

If you want to use the reset pin for I/O then you have to program the chip via a bootloader.

Or use high-voltage programming, although I admit that's something most Arduinoites would not be into.

Rob

12

floresta:

But why do I need to use reset w/ Arduino ISP? (which I will be using).

You cannot program the chip using the ISP (ICSP in Arduinospeak) pins without the reset function enabled. If you want to use the reset pin for I/O then you have to program the chip via a bootloader.

Don

Even with the bootloader, without a reset function the arduino auto-reset-before-uploading function will not work, so you will be trying to plug it into the USB (to get the power-up reset to bootloader) at just the right time to catch the upload handshake. I don't think that would work often enough before one gives up in disgust.