44.4.5 Continuous Serial Communications Clock
The module 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 MCR. Enabling this
bit generates the Continuous SCK regardless of the MCR[HALT] bit status. Continuous
SCK is valid in all configurations.
Continuous SCK is only supported for CPHA=1. 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:
• When the module is in SPI configuration, CTAR0 is used initially. At the start of
each SPI frame transfer, the CTAR specified by the CTAS for the frame is used.
• In all configurations, the currently selected CTAR remains in use until the start of a
frame with a different CTAR specified, or the Continuous SCK mode is terminated.
It is recommended to keep the baud rate the same 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 module is put into the
External Stop mode or Module Disable mode.
Enabling Continuous SCK disables the PCS to SCK delay and the Delay after Transfer
(t
DT
) is fixed to one SCK cycle. The following figure is the timing diagram for
Continuous SCK format with Continuous Selection disabled.
NOTE
In Continuous SCK mode, for the SPI transfer CTAR0 should
always be used, and the TX FIFO must be cleared using the
MCR[CLR_TXF] field before initiating transfer.
Chapter 44 Serial Peripheral Interface (SPI)
K22F Sub-Family Reference Manual , Rev. 3, 7/2014
Freescale Semiconductor, Inc.
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