Hitachi H8S/2338 Series Hardware Manual page 212

Activation by Falling Edge on DREQ Pin: DREQ pin falling edge detection is performed in
synchronization with DMAC internal operations. The operation is as follows:
[1] Activation request wait state: Waits for detection of a low level on the DREQ pin, and switches
to [2].
[2] Transfer wait state: Waits for DMAC data transfer to become possible, and switches to [3].
[3] Activation request disabled state: Waits for detection of a high level on the DREQ pin, and
switches to [1].
After DMAC transfer is enabled, a transition is made to [1]. Thus, initial activation after transfer
is enabled is performed on detection of a low level.
Activation Source Acceptance: At the start of activation source acceptance, a low level is
detected in both DREQ pin falling edge sensing and low level sensing. Similarly, in the case of an
internal interrupt, the interrupt request is detected. Therefore, a request is accepted from an
internal interrupt or DREQ pin low level that occurs before execution of the DMABCRL write to
enable transfer.
When the DMAC is activated, take any necessary steps to prevent an internal interrupt or DREQ
pin low level remaining from the end of the previous transfer, etc.
Internal Interrupt after End of Transfer: When the DTE bit is cleared to 0 at the end of a
transfer or by a forcible termination, the selected internal interrupt request will be sent to the CPU
or DTC even if DTA is set to 1.
Also, if internal DMAC activation has already been initiated when operation is forcibly
terminated, the transfer is executed but flag clearing is not performed for the selected internal
interrupt even if DTA is set to 1.
An internal interrupt request following the end of transfer or a forcible termination should be
handled by the CPU as necessary.
Channel Re-Setting: To reactivate a number of channels when multiple channels are enabled, use
exclusive handling of transfer end interrupts, and perform DMABCR control bit operations
exclusively.
Note, in particular, that in cases where multiple interrupts are generated between reading and
writing of DMABCR, and a DMABCR operation is performed during new interrupt handling, the
DMABCR write data in the original interrupt handling routine will be incorrect, and the write may
invalidate the results of the operations by the multiple interrupts. Ensure that overlapping
DMABCR operations are not performed by multiple interrupts, and that there is no separation
between read and write operations by the use of a bit-manipulation instruction.
Also, when the DTE and DTME bits are cleared by the DMAC or are written with 0, they must
first be read while cleared to 0 before the CPU can write a 1 to them.
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