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My circuit already has a crystal that outputs a 12MHz signal to the MCU, which then PLL it to 48MHz internally for use. Since I also need 48MHz on the MAX 10 10M02 FPGA, I was wondering why not simply use the same crystal signal for both.
First question is, can I use the same signal from the same crystal for both devices, the MCU and the FPGA? I can't see why not, but then again I'm still an Altera Pupil :oops:
And if yes, are there any disadvantages in sending a 12MHz signal to the FPGA and PLL it x4 to the main clock versus sending a signal at already 48MHz?
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1) By 'crystal' I assume you mean a crystal oscillator (an active 4pin device that outputs an LVCMOS, for example, signal). If so, yes it would work. You could route the 48MHz signal to both the MCU and the FPGA.
If you really mean a discrete 2pin passive crystal, the answer is NO. That will not work.
2) Sending 12MHz to the FPGA and having it do 4X multiplication to 48MHZ, and then using that internal to the FPGA, and sending 12MHz signal to the MCU via a pin should work as well (the 12MHz could be a copy of the 12MHz input, or the 48MHz divided by four). Would just have to make sure the I/O levels are compatible.
The one gotcha might be on reprogramming of the FPGA; the output to the MCU will likely be disabled during FPGA programming. So the MCU clock would disappear then. This may or may not be a problem, it depends on your application (ie, if the MCU is responsible for reprogramming the FPGA).
So I suspect using a 12MHz external crystal oscillator, and sending the output to both the MCU and FPGA (where it then gets multiplied by 4X internally) is probably the best overall solution. A crystal oscillator can certainly drive two loads.
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