I am trying to make an eval-board MCB2368 work at 19.968MHz. According to the user manual, 275<Fcco<550MHz. However, at the errata sheet version 1.1 for LPC2368 is said that the limit for Fcco is 290MHz. So, I realize we have the range 275<Fcco<290MHz for LPC2368.
The LPC2368 documentation says also that 1<N<256 and 1<M<32768.
To match my needs, I found the values N=125, M=1456 (for these values I got Fcco=279.552MHz), both inside the respect range.
My problem is that for any value of N bigger than 37 the PLL doesn't work.
Some one can explain me if I am doing something wrong or is it really missing information at LPC2300 documentation?
The formula for PLL output is FCCO = (2 × M × FIN) / N and M and N starts from 1 So Whatever value you write to M and N an '1' should be added while calculation.
So with 19.968Mhz Fin FCCO = 2 * 1457 * 19.968Mhz / 126 = 461.799 Mhz
Something wrong ?
Suvidh
Sorry, I would mean Cclk=19.968MHz.
The eval-board MCB2368 has already a 12MHz oscilation crystal. So, FIN=12MHz.
My values: M = 1456 N = 125 clk divider = 14
Using the formula: FCCO = (2 × M × FIN) / N FCCO = (2 × 1456 × 12) / 125 = 279.552MHz Cclk = Fcco / (clk divider) = Fcco / 14 = 19.968MHz
My problem: When N>37 PLL doesn't work. And I need N=125.
Hi, At higher oscillator frequencies, in the MHz range, values of M from 6 through 512 are allowed. Some higher values are supported but at low FIN in Khz.
OK, with this information I see M is out of the range. Thank you.
But, for 6<M<512, N value is also problematic and in low values.
For example, using a MCB2368: M = 419 N = 36 FIN = 12MHz I got Cclk = 19.9523MHz and it works.
But: M = 440 N = 38 FIN = 12MHz I got Cclk = 19.8496MHz and it doesn't work.
Any reason for that?