Where are you getting the +31 from? Is this in the User's Manual?
No in user's manual. The +31 is rounding up purpose.
Computations are for mathematicians. Real engineers measure the
actual frequency. ;-) Agree.
If you change the " - 31" to " - 42", your computations will result in
a different divisor.
Why are you subtracting what appears to be an arbitrary number (31)?
Can't use 42 when the user's manual spec the divisor is 32. The "- 31" comes from counter = ((bus_freq / baudrate) + 31) / 32. Using the same calculation as mentioned here to find out bus_freq. bus_freq = ((counter * 32) - 31) * baudrate.
Are you sure your input frequency is 140MHz and not 144MHZ??? Have
you read the itty-bitty
numbers on the crystal? Have you put a scope on it and actually
measured it? Never trust
those hardware people. ;-)
Actually, the bus freq is 70Mhz, 140Mhz is cpu freq (scope measurement). The input freq (crystal/oscillator) not always 1 to 1 ratio. Take M5253EVBE for example, the input clk is 11Mhz. Then, it multiply up and divide down for CPU and bus frequency.
Looking at the MFC5407 (arbitrarily, not knowing what processor you
have), the dividers are
as expected and *do not* match your "- 31" calculations. They have a simple example and simple math that matches
(no "- 31").
Hmmm, the MFC5407 has a max CLKIN (and thus bus clock) of 54MHz. You
must have a different
flavor. MFC548x maxes at 50MHz. What processor are you using?
52x2, 532x, 547x_8x, 5445x. The 5253EVB is having the issue where the calculation does not apply if baudrate is 115200.
Ahh, MFC5249 has a 140MHz bus. Still is a straight forward divide-by
n*32, no "- 31". I thought the same too, but the 140Mhz is cpu freq.
Regards, TsiChung