Forum Discussion
Altera_Forum
Honored Contributor
9 years ago --- Quote Start --- @ak6dn Doesn't work like that... The first multiply/divide counter (M/N) sets the VCO frequency. The VCO frequency must be at least 600MHz. The lowest frequency the PLL output will do is around 1.6MHz. For lower clock frequencies, this should be done in logic, either to make a clock enable, or a low frequency clock as I said in my earlier post. --- Quote End --- That is true for a single, direct PLL output clock. However, for Cyclone V you can use 'cascade counter mode' where you take pll_output_0 at 8.0MHz, for example, and use that as the source clock to pll_output_1 in cascade mode, and then set the divisor to (in this case 250) to generate 8MHz/250 = 32KHz on pll_output_1. It takes two PLL outputs to do this, but it allows for much higher clock division ratios (ie, up to 512*512). Reference, ALTPLL IP User Guide, version 2014-08-18, page 28: --- Quote Start --- pll output counter cascading
In 28 nm devices, a C-counter input can be either a VCO output or the cascaded output of a neighboring C-counter. Cascading C-counters increase the possible division factor, enabling very low frequency PLL output clocks. --- Quote End --- In the QSYS IP wizard, select two PLL clock outputs, and tick the 'cascade mode' box on output 0 to feed it into output 1 source. Ok, here it is in Quartus 16.0: http://www.alteraforum.com/forum/attachment.php?attachmentid=12556&stc=1