Programmable Devices
CPLDs, FPGAs, SoC FPGAs, Configuration, and Transceivers
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19974 Discussions

Center aligned output clock for ALTLVDS_TX in external PLL mode (Arria V)


I am using the ALTLVDS_TX core for transmitting differential data that was previously received by the ALTLVDS_RX core. The target device is an Arria V FPGA. The input/output data rate is 800 Mbps (400 MHz DDR clock).


To avoid having an asynchronous clock domain crossing between RX and TX I am using the ALTLVDS cores in external PLL mode as described in UG-MF9504. The data received by the ALTLVDS_RX core is center-aligned (as opposed to edge-aligned which is the case discussed in UG-MF9504).


As suggested by the user guide, I first generated the ALTLVDS_RX and ALTLVDS_TX cores using the internal PLL mode to determine the PLL parameters (frequency, phase shift, duty cycle) for the required PLL clocks. I then switched to external PLL mode and instantiated a PLL with the corresponding parameters.


The problem is that in external PLL mode there is no way to specify a phase shift for the ALTLVDS_TX output clock (tx_outclock) which is required to that the output data is center aligned with the output clock.


I added an additional output clock to the PLL with the desired frequency (400 MHz) and phase shift with respect to the ALTLVDS_TX output data. The resulting wave-form is correct according to the simulation. However, when measuring on the board the TX clock and data are not center aligned.


Is there a way to achieve center-aligned output clock when using the ALTLVDS_TX core in external PLL mode?

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2 Replies



To achieve center-aligned adjust the phase shift of external pll.

Can you recheck the phase shift setting of external pll.

Do refer below links for more information.


Let me know if this has helped resolve the issue you are facing or if you need any further assistance.


Best Regards,

Anand Raj Shankar

(This message was posted on behalf of Intel Corporation)






when generating a ALTLVDS_TX using the MegaWizard, it is not possible to specify a phase shift is the "Use external PLL" option is selected.


For the PLL I am using the phase shifts reported in the resource report when using internal PLL for my setup (as suggested in this document (see page 51)).


Is there any example PLL configuration for ALTLVDS_TX with center-aligned clock/data?