M306V2EEFS datasheet(77/271 Pages) RENESAS

77 / 271 page

Rev.1.40

Oct 06, 2004 page 77 of 269

M306V2ME-XXXFP, M306V2EEFP

2.9.3 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.

2.9.4 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

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 sig=ls.

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