Information

3. Enable error interrupts in the EEI register if so desired.
4. Write the 32-byte TCD for each channel that may request service.
5. Enable any hardware service requests via the ERQ register.
6. Request channel service via either:
• Software: setting the TCDn_CSR[START]
• Hardware: slave device asserting its eDMA peripheral request signal
After any channel requests service, a channel is selected for execution based on the
arbitration and priority levels written into the programmer's model. The eDMA engine
reads the entire TCD, including the TCD control and status fields, as shown in the
following table, for the selected channel into its internal address path module.
As the TCD is read, the first transfer is initiated on the internal bus, unless a
configuration error is detected. Transfers from the source, as defined by TCDn_SADDR,
to the destination, as defined by TCDn_DADDR, continue until the number of bytes
specified by TCDn_NBYTES are transferred.
When the transfer is complete, the eDMA engine's local TCDn_SADDR,
TCDn_DADDR, and TCDn_CITER are written back to the main TCD memory and any
minor loop channel linking is performed, if enabled. If the major loop is exhausted,
further post processing executes, such as interrupts, major loop channel linking, and
scatter/gather operations, if enabled.
Table 21-105. TCD Control and Status fields
TCDn_CSR field
name
Description
START Control bit to start channel explicitly when using a software initiated DMA service (Automatically
cleared by hardware)
ACTIVE Status bit indicating the channel is currently in execution
DONE Status bit indicating major loop completion (cleared by software when using a software initiated
DMA service)
D_REQ Control bit to disable DMA request at end of major loop completion when using a hardware initiated
DMA service
BWC Control bits for throttling bandwidth control of a channel
E_SG Control bit to enable scatter-gather feature
INT_HALF Control bit to enable interrupt when major loop is half complete
INT_MAJ Control bit to enable interrupt when major loop completes
The following figure shows how each DMA request initiates one minor-loop transfer, or
iteration, without CPU intervention. DMA arbitration can occur after each minor loop,
and one level of minor loop DMA preemption is allowed. The number of minor loops in
a major loop is specified by the beginning iteration count (BITER).
Initialization/application information
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
384 Freescale Semiconductor, Inc.