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The terminal didn’t produce any unreadable characters at least. Just for fun I tried the code at a baud rate of 3840 as well. I tested all above baud rates and they seem to be working as intended.
ATTINY USI UART HOW TO
Now we know how to calculate the theoretical limits, we can calculate this as well for chips running at 8 and 16 MHz. However 8 times faster than the original code. The nearest lowest well-used baud rate is therefore 19200. Dividing 1 MHz by 41 gives us a potential baud rate of 24390. It can take however longer than this if the previous instruction before the interrupt was a multi-cycle instruction as this instruction will be finished before the interrupt. This consists of 4 clock cycles for our interrupt to fire, 3 to jump to the interrupt code and 34 to execute the code itself. So our code must be done preparing the second packet and placing it in the USI before the clock has ticked.Īfter compiling the code as it is right now and disassembling it afterwards, I calculated that it takes (at least) 41 clock cycles before we have our next packet ready. However in the mean time the clock is still running. We prepare the second packet when the overflow interrupt has fired. For sending a byte we need to send two packets (again: read the article). So this is definitely not the limiting factor. The USI can be clocked at a maximum speed of CLK/2. While interference also wasn’t a problem anymore, it was time to figure out what my theoretical limit was for the baud rate when running at 1 MHz. Either way, this was solved by pulling all unused pins down to ground in the setup. It might have something to do with the fact I don’t have any grounded power outlets in my house or maybe because I am running everything from USB ports out of my PC. Why this is happening, is still unknown to me.
ATTINY USI UART SERIAL
As soon as I touched the pin myself or as soon as I touched any ground, the serial was working fine. To my suprise this was caused by interference on the reset pin of the ATtiny85. The terminal was spitting out garbage again. Already an improvement of 4 times! However increasing the baud rate to 19200 caused problems. Now I could already increase the baud rate to 9600. GTCCR = 0 // Disable sync mode Can we go even faster?
ATTINY USI UART FULL
OCR0A = FULL_BIT_TICKS // Trigger every full bit width TCCR0B = CLOCKSELECT // Set prescaler to clk or clk /8 GTCCR = (1< This can be done by setting the TSM and PSM0 bits in GTCCR and clearing them when we are done. Adjusting the timer properties while it is running is somewhat dangerous so I improved the code to stop the timer while we set all the properties and start it only after everything is set. This timer is also used by the Arduino Core if you use that, so there will be collision there as described in the post. So what else was going wrong here?Īs Timer0 is used as the clock for the USI, I assumed the problem must be in there somewhere. Even we purely bit bang in software we can manage to do this within 208 clock cycles. If we simply divide the clock speed by the baud rate, we see that the chip has 208 clock cycles per bit available. However increasing it to 4800, made the terminal spit out garbage. After fixing a minor bug in the example code, I had a nice working UART at a baud rate of 2400. My goal was to see how far I can push the baud rate when the chip is clocked at it’s lowest clockspeed. So I implemented the USI Serial Send solution using an ATtiny85 clocked at 1 MHz. Also I will be limiting myself to the transmitting side only. In this post I will assume you have read the article so I won’t go into much detail of the implementation. Using the USI has the advantage that it is possible to achieve higher baud rates and less CPU overhead than when using a pure software serial. Using this method, it is relatively easy to use the USI in the ATtiny to accomplish an UART as the ATtiny doesn’t have an UART interface by itself. Recently I stumbled across this post by Mark Osborne.
ATTINY USI UART SOFTWARE