Cirrus Logic EP93xx - Manual (Page 718)

23-6 DS785UM1
Copyright 2007 Cirrus Logic
Synchronous Serial Port
EP93xx User’s Guide
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Figure 23-4. Motorola SPI Frame Format (Continuous Transfer)
with SPO=0 and SPH=0
In this configuration, during idle periods:
• the SCLKOUT signal is forced LOW
• SFRMOUT is forced HIGH
• the transmit data line SSPTXD is arbitrarily forced LOW
• when the SSP is configured as a master, the SSPCTLOE line is driven LOW, enabling
the SCLKOUT pad (active LOW enable)
• when the SSP is configured as a slave, the SSPCTLOE line is driven HIGH, disabling
the SCLKOUT pad (active LOW enable).
If the SSP is enabled and there is valid data within the transmit FIFO, the start of
transmission is signified by the SFRMOUT master signal being driven LOW. This causes
slave data to be enabled onto the SSPRXD input line of the master. The master SSPTXD
output pad is enabled.
One half SCLKOUT period later, valid master data is transferred to the SSPTXD pin. Now
that both the master and slave data have been set, the SCLKOUT master clock pin goes
HIGH after one further half SCLKOUT period.
The data is now captured on the rising edges, and is propagated on the falling edges, of the
SCLKOUT signal.
In the case of a single word transmission, after all bits of the data word have been
transferred, the SFRMOUT line is returned to its idle HIGH state one SCLKOUT period after
the last bit has been captured.
However, in the case of continuous back-to-back transmissions, the SFRMOUT signal must
be pulsed HIGH between each data word transfer. This is because the slave select pin
freezes the data in its serial peripheral register and does not allow it to be altered if the SPH
bit is logic zero. Therefore the master device must raise the SFRMIN pin of the slave device
between each data transfer to enable the serial peripheral data write. On completion of the
continuous transfer, the SFRMOUT pin is returned to its idle state one SCLKOUT period after
the last bit has been captured.
M SB LSB LS B M S B
4 to 16 bits
M SB LSB LS B M S B
SSPTXD
SSPOE (=0)
SSPRXD
SFRMOUT /
SFRMIN
SCLKIN
SCLKOUT /