When TBCTL is modified with a MOV instruction, the contents of TBR may be cleared, even when TBCLR is not set.
Workaround
Use BIS or BIC instructions to modify TBCTL.
Note:
A DMA transfer must not occur while these BIS and BIC instructions execute. This can be prevented by disabling the DMA prior to these
instructions or by using the DMAONFETCH bit to align DMA transfers to instruction fetch boundaries.