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I have an Arduino Uno SMD and an attiny45, which I want to use as a sensor communicating via i2c. For a first step in finding out how to work with the attiny45 I just want to use the attiny as controller for a blinking led.

When first setting up the Arduino as "Arduino as ISP" some questions came up:

  1. I found a lot of different information about wheter or not to use a capacitor between RESET and GND on the Arduino. Also the capacity differs between 0.1uF and 10uF. Unfornately, I found not a single comment on why this capacitor is needed (or not) and which capacities are suitable. All tutorials used electrolytic capacitors (which I only have in 100uF at hand). I have 10nF and 0.1uF at hand but only the film capacitor type. It would be brilliant if some could help me with deciding if I should use a capacitor, which one is suitable and what's the reason for using it.

  2. My second question is about the clock rate I should set in the Arduino IDE. I added the board package from https://github.com/SpenceKonde/ATTinyCore to my Arduino IDE, which offers a large variety of clock rates in the tools dropdown menu. In some tutorials they mentioned that it's crucial to pick the correct clock speed to avoid wrecking the attiny. But, unfortunately, the mentioned clock speeds vary between "1 MHz (internal)" and "8 MHz (internal)". In the attiny45 datasheet (http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-2586-AVR-8-bit-Microcontroller-ATtiny25-ATtiny45-ATtiny85_Datasheet.pdf) I found the "Calibrated Internal Oscillator" to be the clock set up in freshly shipped chips, which I understand to operate at 8 MHz. But, the datasheet also says, that the CKDIV8 fuse is set on shipment, which leads to a clock prescale value of 8 and to a clock rate of 1 MHz. I'm highly confused... Maybe someone here can help me to find the correct clock rate to set.

I hope I didn't forget any important information. If so, please let me know.

Kind regards and thank you in advance, Matthias

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  1. The capacitor is meant to prevent the programming Arduino from being reset. Normally, when you open the serial connection, the Arduino gets reset and first starts into the bootloader (which would be used to program it). But when using the Arduino as ISP programmer, you don't want to run the bootloader of the programmer. You want to let the programmer sketch work as intended. Thus you want to prevent the programmer from resetting on serial connection. The capacitor between Reset and ground holds the 5V level of the reset pin for the time, that the reset pulse from the USB Serial chip is trying to pull the pin to low. So it prevents the reset from happening.

    The value and type of the capacitor is not critical here. I always use a 47uF bipolar electrolyte capacitor, because I have a bunch of them just laying around. The value just shouldn't be unreasonable small or big. The mentioned range of yours is just giving you a hint in which order the value should be. Also you can simply test, if the reset happens with your capacitor. You cannot break things with a wrong value here.

  2. Which clock frequency to use is up to your exact requirements. There is no "this is the best for all occasions"-frequency. Also you are not wrecking the Attiny with any of the clock speeds. Though you should note, that choosing an external clock source means, that you actually need to provide that clock source for the ATTiny to work. The fuses can be reprogrammed, if you have chosen a wrong frequency by accident.

    The datasheet states just as you wrote. The interal oscillator is by default set to 8MHz, while the prescaler divides that down to 1MHz as the main CPU clock. So by default the ATTiny runs on 1MHz. You can change that, if you like.

    For my Attiny85s I always used internal 8MHz as the setting (the option, that appears in the IDE menu). That is the easiest way (internal means you need no external components for the clock), while being the fastest speed you can get with the internal clock, I think.

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  • Great, that helped me a lot. Thank you very much @chrisl – Matthias Jung Jul 10 at 11:50
  • The reset pulse goes through a 0.1 µF cap, so a 0.1 µF holding cap is really borderline. I would aim for ≥ 0.47 µF, or 0.22 µF as the very minimum. – Edgar Bonet Jul 10 at 11:58

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