Freescale Semiconductor MPC5553, Reference Manual

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MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
Freescale Semiconductor 19-63
• Its commands are bound for an external command buffer that is not full, and it is the highest priority
triggered CFIFO sending commands to an external buffer that is not full.
A triggered CFIFO with commands bound for a certain command buffer consecutively transfers its
commands to the buffer until one of the following occurs:
• An asserted end of queue bit is reached.
• An asserted pause bit is encountered and the CFIFO is configured for edge trigger mode.
• CFIFO is configured for level trigger mode and a closed gate is detected.
• In case its commands are bound for an internal command buffer, a higher priority CFIFO that uses
the same internal buffer is triggered.
• In case its commands are bound for an external command buffer, a higher priority CFIFO that uses
an external buffer is triggered.
The prioritization logic of the eQADC, depicted in Figure 19-37 , is composed of three independent
submodules: one that prioritizes CFIFOs with commands bound for ADC0, another that prioritizes
CFIFOs with commands for ADC1, and a last one that prioritizes CFIFOs with commands for external
command buffer 2 and buffer 3. As these three submodules are independent, simultaneous commands to
ADC0, to ADC1, and to eQADC SSI transmit buffer are allowed. The hardware identifies the destination
of a command by decoding the EB and BN bits in the command message (see
Section 19.4.1.2, “Message
Format in eQADC ,” for details).
NOTE
Triggered but empty CFIFOs, underflowing CFIFOs, are not considered for
prioritization. No data from these CFIFOs will be sent to either of the
on-chip ADCs or to either of the external command buffers, nor will they
stop lower priority CFIFOs from transferring commands.
Whenever ADC0 is able to receive new commands, the prioritization submodule selects the
highest-priority triggered CFIFO with a command bound for ADC0, and sends it to the ADC. In case
ADC0 is able to receive new entries but there are no triggered CFIFOs with commands bound for it,
nothing is sent. The submodule prioritizing ADC1 usage behaves in the same way.
When the eQADC SSI is enabled and ready to start serial transmissions, the submodule prioritizing
eQADC SSI usage writes command or null messages into the eQADC SSI transmit buffer, data written to
the eQADC SSI transmit buffer is subsequently transmitted to the external device through the eQADC SSI
link. The submodule writes commands to the eQADC SSI transmit buffer when there are triggered CFIFOs
with commands bound for not-full external command buffers. The command written to the transmit buffer
belongs to the highest priority CFIFO sending commands to an external buffer that is not full. This implies
that a lower priority CFIFO can have its commands sent if a higher priority CFIFO cannot send its
commands due to a full command buffer. The submodule writes null messages to the eQADC SSI transmit
buffer when there are no triggered CFIFOs with commands bound for external command buffers, or when
there are triggered CFIFOs with commands bound for external buffers but the external buffers are full. The
eQADC monitors the status of the external buffers by decoding the BUSY fields of the incoming result
messages from the external device (see
Section , “ Result Message Format for External Device
Operation ,” for details).