M16C30_02 Datasheet, PDF (75/178 Pages) Renesas Technology Corp – SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER

DMAC
Mitsubishi microcomputers
M16C / 30 Group
SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER
DMA enable bit
Setting the DMA enable bit to “1” makes the DMAC active. The DMAC carries out the following operations
at the time data transfer starts immediately after DMAC is turned active.
(1) Reloads the value of one of the source pointer and the destination pointer - the one specified for the
forward direction - to the forward direction address pointer.
(2) Reloads the value of the transfer counter reload register to the transfer counter.
Thus overwriting “1” to the DMA enable bit with the DMAC being active carries out the operations given
above, so the DMAC operates again from the initial state at the instant “1” is overwritten to the DMA
enable bit.
DMA request bit
The DMAC can generate a DMA transfer request signal triggered by a factor chosen in advance out of
DMA request factors.
DMA request factors include the following.
* Factors effected by using the interrupt request signals from the built-in peripheral functions and software
DMA factors (internal factors) effected by a program.
* External factors effected by utilizing the input from external interrupt signals.
For the selection of DMA request factors, see the descriptions of the DMA0 factor selection register.
The DMA request bit turns to “1” if the DMA transfer request signal occurs regardless of the DMAC's state
(regardless of whether the DMA enable bit is set to “1” or “0”). It turns to “0” immediately before data
transfer starts.
In addition, it can be set to “0” by use of a program, but cannot be set to “1”.
There can be instances in which a change in DMA request factor selection bit causes the DMA request bit
to turn to “1”. So be sure to set the DMA request bit to “0” after the DMA request factor selection bit is
changed.
The DMA request bit turns to “1” if a DMA transfer request signal occurs, and turns to “0” immediately
before data transfer starts. If the DMAC is active, data transfer starts immediately, so the value of the
DMA request bit, if read by use of a program, turns out to be “0” in most cases. To examine whether the
DMAC is active, read the DMA enable bit.
Here follows the timing of changes in the DMA request bit.
(1) Internal factors
Except the DMA request factors triggered by software, the timing for the DMA request bit to turn to “1” due
to an internal factor is the same as the timing for the interrupt request bit of the interrupt control register to
turn to “1” due to several factors.
Turning the DMA request bit to “0” due to an internal factor is timed to be effected immediately before the
transfer starts.
(2) External factors
________
An external factor is a factor caused to occur by the leading edge of input from the INT0 pin.
________
Selecting the INT0 pins as external factors using the DMA request factor selection bit causes input from
these pins to become the DMA transfer request signals.
The timing for the DMA request bit to turn to “1” when an external factor is selected synchronizes with the
signal's edge applicable to the function specified by the DMA request factor selection bit (synchronizes
________
with the trailing edge of the input signal to INT0 pin, for example).
With an external factor selected, the DMA request bit is timed to turn to “0” immediately before data
transfer starts similarly to the state in which an internal factor is selected.
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