Why does the AS_CLK frequency unexpectedly change to 25MHz when the Mailbox Client Intel® FPGA IP or the Serial Flash Mailbox Client Intel® FPGA IP accesses a flash memory device when using Intel® Stratix® 10 devices? - Why does the AS_CLK frequency unexpectedly change to 25MHz when the Mailbox Client Intel® FPGA IP or the Serial Flash Mailbox Client Intel® FPGA IP accesses a flash memory device when using Intel® Stratix® 10 devices? Description Due to a problem in Intel® Quartus® Prime Pro Edition software version 20.3 and earlier, Active Serial configuration clock (AS_CLK) frequency is unexpectedly changed to 25MHz if QSPI_SET_CS command is used when the Mailbox Client Intel® FPGA IP or the Serial Flash Mailbox Client Intel® FPGA IP accesses a flash memory device when using Intel® Stratix® 10 devices. AS_CLK frequency can be specified by Active serial clock source menu in Configuration category of Device and Pin Options dialog box. This setting should be applied to AS_CLK frequency for not only Active Serial configuration but also for those IPs operation, but once QSPI_SET_CS command is used, AS_CLK frequency is incorrectly changed to 25MHz. There is no way to revert AS_CLK frequncy back to the frequency specified in the Active Serial clock source menu. Resolution To avoid this problem, do not use QSPI_SET_CS command. Although the Mailbox Client Intel® FPGA IP User Guide and the Serial Flash Mailbox Client Intel® FPGA IP User Guide instruct to use QSPI_SET_CS command before one of the quad SPI operations, it is not necessary when only a flash device is connected to an Intel Stratix 10 device and the chip select of the flash device is connected to nCS[0] of the Intel Stratix 10 device. There is no workaround when multiple flash devices are connected and QSPI_SET_CS command is required to select one of those flash devices. This problem is fixed starting with the Intel® Quartus® Prime Pro Edition software version 21.1. Custom Fields values: ['novalue'] Troubleshooting 1508231819 False ['novalue'] ['FPGA Dev Tools Quartus® Prime Software Pro'] 21.1 20.3 ['Stratix® 10 FPGAs and SoCs'] ['novalue'] ['novalue'] ['novalue']