12/29/2023 0 Comments Atmega328p arduino pinout![]() ![]() When the RESET pin on the 328P is low, all the pins (except for the ICSP pins, I believe) are set to a high impedance state, effectively killing the serial pins. We just want to pulse the pin very, very briefly, so that the bootloader can listen to SERIAL_Rx and determine if a new sketch is being sent. It’s important though, that we don’t hold the 328P in reset. That’s why your sketch always restarts when you open a new serial monitor window! Any time the 16U2 establishes a serial connection, the DTR is brought low, which causes a reset and cues the bootloader. The Arduino designers take advantage of this process, by using the DTR signal to issue a reset from the 16U2 to the 328P. On the old DB9 serial cables we once used to connect modems and crappy label printers to our Pentium PCs, DTR was pin 4 in that cable. When DTR is high, it says, “Hey, I’m ready for all the data you can send.” When DTR is low, it says, “WOAH! Gimme a break, I need to process some stuff so hold off on the communication.” Part of the USART communications standard allows for something called “flow control” which allowed a computer on one end of a serial connection to start or stop the serial conversation by setting the logic state of a pin called “Data-Terminal-Ready” or DTR (there are other variants on this principle). The USB goes to the 16U2, which translates it into USART, and handles the interaction to the 328P. ![]() When we talk to the 328P from our computer, we do it over USB. On the UNO R3 (and most current Arduino variants), an “auto-reset” feature has been implemented to take care of this for us. It would be annoying if we had to press the reset button manually every time we corrected some little bug in our code and sent it back to the chip. The key in that sequence for us is the “when the Arduino gets reset, the bootloader checks” part. If no program is waiting, the bootloader drops off and allows the serial connection to run as normal. If a new program is there, the bootloader overwrites the existing sketch with the new sketch and hands control over to it. When the 328P on your Arduino gets reset, the bootloader (currently Optiboot, I believe) checks to see if there is a new program waiting to be installed by checking if anything is waiting on the serial line. Now we’ll pull apart the Data Terminal Read (DTR) and RESET spaghetti.īuild An Arduino UNO R3 from Scratch Table of Contents In part 10, we opened up the ATMEGA328P can of worms, looking at bypass caps, serial termination and the ceramic resonator.
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