Honored ContributorI'm trying to create a phase stepping reconfigurable PLL. The granularity of the phase stepping is 1/8 of the VCO frequency. I need smaller step and I'll need a higher VCO frequency. I can only get 800MHz VCO frequency when I run the system clock at 266.666MHz output from the PLL. How do I get higher VCO frequency, eg. 1000MHz?
Honored ContributorAnother question will be, can I get higher than 1000MHz VCO frequency?
Honored ContributorWhat part's PLL are you using? And if it's a Stratix-family part, are you asking about Enhanced or Fast PLLs?
Honored ContributorThe max spec for SII PLLs is 1040 MHz even in fastest speed grade. You can't get higher than 1040 MHz VCO unless you lie to the tool about your input frequency. E.g., you set for 100 MHz input with 200 MHz output and 125 ps phase step. This will set VCO = 1000 MHz to achieve the 125 ps, then divide that by 5 to get the 200 MHz output. However, if you set the input frequency to higher than 100 MHz, you will be running the VCO higher than it is specified for.
With your frequency case, 266 MHz output requires a 800 MHz or a 1066 MHz VCO. The latter is what you want but its illegal / out of specification but only by 26 MHz. You can fool the tool and enter that you have 250 MHz output but then run it at 266 MHz on the board....but you are technically running out of spec.Honored ContributorI think it's better to run it at 260MHz then. I can get 1040MHz from the VCO and even if I were to overclock it to 266MHz, it's just running 6MHz more and at the same time for the VCO is only 26MHz.
Honored ContributorYes, this is a sound decision. It gets you over 1000 MHz VCO and still remains in spec.
