Hello Tom,
Could you please let me know which BSP are you using in order to test this issue?, You mentioned you are using the K20DX256M10 device but I don't know which BSP you are using.
Remember you need to rebuild the MQX libraries after change the BSP_ALARM_FREQUENCY.
In addition, please check the below thread and let me know if this helps?
Using the MQX timers
Have a great day,
Sol
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