PXN20RM Datasheet, PDF (977/1376 Pages) Freescale Semiconductor, Inc – PXN20 Microcontroller

Deserial – Serial Peripheral Interface (DSPI)
Switching CTAR registers or changing which PCS signals are asserted between frames while using
continuous selection can cause errors in the transfer. The PCS signal should be negated before CTAR is
switched or different PCS signals are selected.
30.4.8.6 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 Figure 30-35, time A shows the one clock interval. Time B is user programmable from a
minimum of two system clocks. Refer to Section 30.3.2.3, DSPI Clock and Transfer Attributes Registers
0–7 (DSPI_CTARn).
System Clock
PCS
Frame 0
Frame 1
SCK
CPOL = 0
AB
CPOL = 1
Figure 30-35. Polarity Switching Between Frames
30.4.9 Continuous Serial Communications Clock
The DSPI provides the option of generating a continuous SCK signal for slave peripherals that require a
continuous clock.
Continuous SCK is enabled by setting the CONT_SCKE bit in the DSPI_MCR. Continuous SCK is valid
in all configurations.
Continuous SCK is only supported for CPHA = 1. Setting CPHA = 0 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:
When the DSPI is in SPI configuration, CTAR0 shall be used initially. At the start of each SPI
frame transfer, the CTAR specified by the CTAS for the frame shall be used.
• When the DSPI is in DSI configuration, the CTAR specified by the DSICTAS field shall be used
at all times.
• When the DSPI is in CSI configuration, the CTAR selected by the DSICTAS field shall be used
initially. At the start of a SPI frame transfer, the CTAR specified by the CTAS value for the frame
shall be used. At the start of a DSI frame transfer, the CTAR specified by the DSICTAS field shall
be used.
• In all configurations, the currently selected CTAR shall remain in use until the start of a frame with
a different CTAR specified, or the continuous SCK mode is terminated.
Freescale Semiconductor
PXN20 Microcontroller Reference Manual, Rev. 1
30-51