5.2.4 LBus-I/F DMA request
The LBus-I/F generates the signal HLBDRQ to request the next data transfer.
HLBDRQ can be used as a DMA trigger signal on the Host CPU side in order to
handle data transfers with minimum Host CPU load.
Caution
The DMA request generation operates only for word transfers.
5.2.4.1
LBus-I/F DMA request setup
DMA request setup
The DMA request generation must be initialized as follows:
•
Select the transfer direction by setting the HOSTCONTROL.DRQDIR
bit accordingly.
•
Enable the DMA request operation by setting the
HOSTCONTROL.DRQEN = 1.
5.2.4.2
LBus-I/F DMA request operation
During the data transfer with enabled DMA request the number of transferred
bytes are counted by an internal byte counter. After the 4th byte has been
transferred and the LBus-I/F is ready to deal with new data the DMA request signal
HLBDRQ is asserted.
The time between the transfer of the 4th byte and the activation of HLBDRQ may
vary, depending on the time necessary to transfer the data from the LBus-I/F to
the final destination, or vice versa respectively.
DMA restart
If the DMA request generation is enabled (HOSTCONTROL.DRQEN = 1), the DMA
byte counter can be cleared and restarted by setting
HOSTCONTROL.DRQEN = 1 again.
By this the Host CPU can restarted the DMA transfer if the byte transfer sequence
was interrupted for any reason.
DMA request status
The status of the HLBDRQ signals is reflected in the bit HOSTSTATUS.DREQ.
5.2.4.3
LBus-I/F DMA operation cautions
Following operating cautions must be regarded:
(1)
Only one direction at a time
During DMA operation, only one data transfer direction must be used - either
receive or transmit operation.When the transfer direction is to change,
reconfiguration by writing to the HOSTCONTROL register is mandatory.
(2)
Only 32-bit transfers
Set up the DMA operation only for word transfers. Commands and data in the
DMA stream may have another size, but the total amount of DMA transfers needs
to be a multiple of four bytes.
(3)
No other transfers in between
Once a 4-byte transfer with DMA operation enabled is running the Host CPU may
not issue direct read or write access to the Host-I/F – otherwise the DRQCU
operation will loose the synchronization.
If a DMA transfer needs to be interrupted, first disable the DMA on the Host CPU
and then disable DMA operation by clearing the HOSTCONTROL.DRQEN bit. The
Chapter 5
Host CPU Interface
132
Preliminary User's Manual S19203EE1V3UM00
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