Mitsubishi microcomputers
M16C / 61 Group
DMAC
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 for each channel.
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 DMAi 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 1 or to 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 1 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 INTi pin (i depends on
which DMAC channel is used).
Selecting the INTi 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 each INTi 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.
68
MITSUBISHI [ Mitsubishi Group ]
