Hitachi F-ZTAT H8/3039 Series Hardware Manual page 263

 Buffer transfer at transition from down-counting to up-counting
If the general register value is in the range from H'0000 to T – 1, do not transfer a buffer
register value outside this range. Conversely, when a general register value is outside this
range, do not transfer a value within this range. See figure 8-41.
TCNT3
TCNT4
T
T – 1
Illegal changes
H'0000
H'FFFF
Figure 8-41 Changing a General Register Setting by Buffer Transfer (Caution 2)
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