Hi Johan >> https://github.com/nospam2000/ch341-baudrate-calculation/blob/master/README.md >Interesting! From a very quick glance this looks very promising (although he doesn't seem to handle rounding yet). I'll take a closer look and compare with what I've done. That’s my code. I actually do rounding. It might not be obvious that the '(*2 + 1) / 2’ in ((2UL * CH341_OSC_F) + / (prescaler * priv->baud_rate) + 1UL) / 2UL does a bankers rounding, that why I explained it in the README.MD: https://github.com/nospam2000/ch341-baudrate-calculation#rounding-issues Rounding issues Why not just using the simple formula (CH341_CRYSTAL_FREQ / (prescaler * baud_rate))? Because we are using integer arithmetic and truncating values after the division leads to an error which only goes into the positive direction because the fractional part of divisor is lost. With floating point arithmetic you would do 5/10 bankers rounding to spread the error equally between positive and negative range: divisor = TRUNC(CH341_CRYSTAL_FREQ / (prescaler * baud_rate) + 0.5) which is equal to divisor = TRUNC((10 * CH341_CRYSTAL_FREQ / (prescaler * baud_rate) + 5) / 10) The formula in the code does the same but using dual system integer arithmetic and therefore use *2 +1 /2 instead of *10 +5 /10. > I also noticed that we should always pick the higher base clock if the resulting divisor is odd (and within the valid range). The general rule from the perspective of mathematics is to use the highest possible divider (and therefore the lowest possible prescaler) because that gives the lowest possible relative error in the division. As long as the result of divider*prescaler is the same it doesn’t matter which combination you choose. For example my algorithm prefers 1*208=208 over 2*104=208 of the other algorithms which doesn’t make a difference. At 110 baud there is the only difference between my algorithm and Jonathan’s code for the commonly used baud rates because I pick 512*213 whereas he doesn’t use the 512 prescaler and therefore uses 1024*107 (both values are odd). The reason why my algoorithm and Jonathan’s have different end results is because I use all possible prescaler combinations and he doesn’t use the prescalers 64 and 512 which gives different results for baud rates around 100. Please look at my comparison table at https://github.com/nospam2000/ch341-baudrate-calculation/blob/master/patches/README.MD which is created by the unit test at https://github.com/nospam2000/ch341-baudrate-calculation/blob/master/patches/check_baud_rates_unittest.c baud errOrig errMike errJon pre*divOrig pre*divMike pre*divJon 46 -0.10% -0.10% -0.10% 1024*255 1024*255 1024*255 50 +0.16% +0.16% +0.16% 1024*234 1024*234 1024*234 75 +0.16% +0.16% +0.16% 1024*156 1024*156 1024*156 110 -0.44% +0.03% -0.44% 1024*107 512*213 1024*107 135 -0.22% -0.22% -0.22% 1024*87 512*174 1024*87 150 +0.16% +0.16% +0.16% 1024*78 512*156 1024*78 300 +0.16% +0.16% +0.16% 1024*39 512*78 1024*39 600 +0.16% +0.16% +0.16% 128*156 128*156 128*156 1200 +0.16% +0.16% +0.16% 128*78 64*156 128*78 1800 +0.16% +0.16% +0.16% 128*52 64*104 128*52 2400 +0.16% +0.16% +0.16% 128*39 64*78 128*39 4800 +0.16% +0.16% +0.16% 16*156 16*156 16*156 7200 +0.16% +0.16% +0.16% 16*104 8*208 16*104 9600 +0.16% +0.16% +0.16% 16*78 8*156 16*78 14400 +0.16% +0.16% +0.16% 16*52 8*104 16*52 19200 +0.16% +0.16% +0.16% 16*39 8*78 16*39 31250 +0.00% +0.00% +0.00% 2*192 2*192 2*192 38400 +0.16% +0.16% +0.16% 2*156 2*156 2*156 45450 +0.01% +0.01% +0.01% 2*132 2*132 2*132 56000 +0.13% +0.13% +0.13% 2*107 1*214 2*107 57600 +0.16% +0.16% +0.16% 2*104 1*208 2*104 76800 +0.16% +0.16% +0.16% 2*78 1*156 2*78 100000 +0.00% +0.00% +0.00% 2*60 1*120 2*60 115200 +0.16% +0.16% +0.16% 2*52 1*104 2*52 128000 -0.27% -0.27% -0.27% 2*47 1*94 2*47 153846 +0.00% +0.00% +0.00% 2*39 1*78 2*39 187500 +0.00% +0.00% +0.00% 2*32 1*64 2*32 230400 +0.16% +0.16% +0.16% 2*26 1*52 2*26 250000 +0.00% +0.00% +0.00% 2*24 1*48 2*24 256000 -2.34% -0.27% -0.27% 2*24 1*47 1*47 307200 -2.34% +0.16% +0.16% 2*20 1*39 1*39 460800 +0.16% +0.16% +0.16% 2*13 1*26 2*13 500000 +0.00% +0.00% +0.00% 2*12 1*24 2*12 750000 +0.00% +0.00% +0.00% 2*8 1*16 2*8 857143 -0.00% -0.00% -0.00% 2*7 1*14 2*7 921600 -6.99% +0.16% +0.16% 2*7 1*13 1*13 1000000 +0.00% +0.00% +0.00% 2*6 1*12 2*6 1090909 -8.33% +0.00% +0.00% 2*6 1*11 1*11 1200000 +0.00% +0.00% +0.00% 2*5 1*10 2*5 1333333 -10.00% +0.00% +0.00% 2*5 1*9 1*9 1500000 +0.00% +0.00% +0.00% 2*4 2*4 2*4 2000000 +0.00% +0.00% +0.00% 2*3 2*3 2*3 3000000 +0.00% +0.00% +0.00% 2*2 2*2 2*2 baud means the requested baud rate errOrig is the relative error of the resulting baud rate using the original code errMike is the relative error of the resulting baud rate using my new code errJon is the relative error of the resulting baud rate using Jonathan Olds' code pre/divOrig the values of prescaler and divisor using the original code pre/divMike the values of prescaler and divisor using my new code pre/divJon the values of prescaler and divisor using Jonathan Olds' code Michael
