I ran quite a few tests today. My results may be hard to believe, but I feel that I should report my test results. I tested the ability to output a GCLK on various pins of the SAMD21J17A on my custom-designed board. I did not test all pins that are listed as GCLK outputs in the datasheet's Table 7-1, column 'H'. That's because my board uses I2S, I2C, SPI, and USB, so I didn't bother to test any pins which are in use on my board for those functions. To configure SAMD21 clocks for my tests today, I enabled ALL GCLKs. Since I already had code in place to set the processor to 48 MHz operation, I left that code in place, so my tests ran with the main system clock running at 48 MHz. I tested PA14, PA15, PA22, PA23, PB13, PB14, PB15, and PB23. Here are the raw test results: PA14 = 48MHz .. PA15 = no clock output .. PA22 = 8MHz .. PA23=no clock output .. PB13 = no clock output .. PB14 = 48 MHz .. PB15 = no clock output .. PB23 = no clock output. I couldn't believe the results, so I retested again. In my tests today, which I emphasize again did not cover every pin in the 'H' column of the datasheet's Table 7-1, I saw an output GENCLK signal on even-numbered pins (e.g. PA22) but did not see any clocks output on odd-numbered pins. This pattern made me instantly think of the MUX enable detail about "Odd" and "Even" MUX values, but I double-checked my code which set MUXEN in either the Odd or Even halves of the registers. I am lucky that one of the available pins on my board tested "good", specifically PA22 which I've been using for I2C but I can move the I2C function to other currently unused pins. Even though my tests today were surprising, at least now I can move ahead to revise my board so it will perform as designed. I would be happy to make additional tests which others might suggest.