How to obtain a valid license of FPGA SDK for OpenCL

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Yes; however, you still need a license for Quartus if you are going to target Stratix V or Arria 10. You can use Quartus Lite without a license, but that can only target Cyclone devices.

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Do you mean the Quartus Lite version can use OpenCL for Cyclone V devices? The licensing details document (https://www.altera.com/content/dam/altera-www/global/en_us/pdfs/literature/po/ss-quartus-comparison.pdf)states that OpenCL is

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Supported with the Intel Quartus Prime Pro/Standard Edition software.

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It seems to me that the Lite edition does not explicitly support OpenCL, even for Cyclone V.