DMA Serial Peripheral Interface (DSPI)
MCF52277 Reference Manual, Rev. 1
29-32
Freescale Semiconductor
29.4.4.6
Clock Polarity Switching between DSPI Transfers
If it is desired to switch polarity between non-continuous DSPI frames, the edge generated by the change
in the idle state of the clock occurs one system clock before the assertion of the chip select for the next
frame. In
, time A shows the one clock interval. Time B is user programmable from a
minimum of 2 system clocks. See
Section 29.3.3, “DSPI Clock and Transfer Attributes Registers 0–7
.”
Figure 29-20. Polarity Switching between Frames
29.4.5
Continuous Serial Communications Clock
The DSPI provides the option of generating a continuous DSPI_SCK signal for slave peripherals that
require a continuous clock. Continuous SCK is enabled by setting the DSPI_MCR[CONT_SCKE] bit.
Continuous SCK is only supported if CPHA is set. Clearing CPHA is ignored if the CONT_SCKE bit is
set. Continuous SCK is supported for modified transfer format.
Clock and transfer attributes for the continuous SCK mode are set according to the following rules:
•
DSPI_CTAR0 is used initially. At the start of each SPI frame transfer, the DSPI_CTAR
n
specified
by the CTAS field for the frame is used.
•
The currently selected DSPI_CTAR
n
remains in use until the start of a frame with a different
DSPI_CTAR
n
specified, or the continuous SCK mode is terminated.
It is recommended that the baud rate is the same for all transfers made while using the continuous SCK.
Switching clock polarity between frames while using continuous SCK can cause errors in the transfer.
Continuous SCK operation is not guaranteed if the DSPI is put into module disable mode.
Enabling continuous SCK disables the PCS to SCK delay and the after SCK delay. The delay after transfer
is fixed at one DSPI_SCK cycle.
shows timing diagram for continuous SCK format with
continuous selection disabled.
DSPI_PCS
n
System Clock
DSPI_SCK
Frame 1
Frame 0
CPOL = 0
CPOL = 1
A
B
Содержание MCF52277
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