Hello.
I'm building an input-output board based on MCF52259CAG80 (that's the 144 LQFP one). You may find some information about it here: http://labserver.uv.es/ioboard/ .
I'm still not sure how to setup the clocking. I want working USB (as device only), MCU clocked near its maximum frequency and possibility to use UARTs at standard RS-232 frequencies. My guess it that there is no other way than to connect 48MHz source to USB_ALTCLK, use some crystal multiple of RS freqs and multiply it with PLL to drive internal clock near 80MHz. But I don't like the idea of designated USB clock while I could just connect the XTAL directly. Clocking UARTs from timer inputs isn't really interesting either, because I won't be able to divide that frequency.
My other concern are the connections through resistors. M52259EVB uses resistors on almost all the signals, to reduce EMI as it says. Should I really take EMI into account ? On the other hand, M52259DEMOKIT uses resistors on clock pins only, probably to make sure no clock edge is detected while not all the signals are ready, but I couldn't find any information about it. Is my reasoning right? That's where a decent electrical guide would be helpful.
Thanks, in advice, for any usefull response.
Thanks a lot.
Looks like the 80MHz frequency will give me baudrate close enough to the standard values.
But EMI resistors of 1k seem like a very large value. I'm going to put the 22 Ohm chips, like EVB does, it won't be a problem to change them to larger values if needed.
Hi
As I mentioned, when the signals are used for simple switching operations (often the case with GPIO lines) and are not delivering current, a large serial resistance will not affect operation but will give high EMI filtering levels.
For clock signals the maximum resistance will depend on the frequency and the board capacitance. 22R may also be suitable for matching the line impedance too.
Of course foreseeing a resistor in the lines allows flexibility for all cases.
Regards
Mark