42.6.2.6.1 Limitations of Auto Character Mapping Mode

If ACM is enabled and the Auto Character Mapping Busy bit in the Status register (STATUS.ACMBUSY) is asserted, any write access to the Character Mapping Data register (CMDATA) will be considered as coming from the DMA controller, and therefore handled by the ACM state machine. So if the user accidentally writes CMDATA in this period, the ACM state machine will be corrupted. Consequently, the user should not write to the CMDATA register when ACM is active, and make sure that all CMDATA access is in the period of ACM inactivity.

For this reason, the recommended usage is to first disable ACM, and then update CMINDEX and CMDATA. After that, ACM can be re-enabled safely.