Freescale Semiconductor MPC5553, Reference Manual

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MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
Freescale Semiconductor 19-77
sending new commands to its buffer—see Figure 19-44 . An external command buffer is
considered empty when the corresponding BUSY field in the last result message received from
external device is encoded as “Send available commands - buffer is empty”. Refer to
Section ,
“ Result Message Format for External Device Operation .” The NCF flag becomes asserted
immediately after the eQADC detects that the external buffer in use becomes empty.
NOTE
After the transfer of a command sequence to an external command buffer
starts, the eQADC ignores, for non-coherency detection purposes, the
BUSY fields captured at the end of the first serial transmission. Thereafter,
all BUSY fields captured at the end of consecutive serial transmissions are
used to check the fullness of that external command buffer. This is done
because the eQADC only updates its external ADC command buffer status
record when it receives a serial message, resulting that the record kept by
the eQADC is always outdated by, at least, the length of one serial
transmission. This prevents a CFIFO from immediately becoming
non-coherent when it starts transferring commands to an empty external
command buffer. Refer to
Figure 19-42 for an example.
Figure 19-42. External Command Buffer Status Detection
at Command Sequence Transfer Start
After a command sequence starts to be transferred, the eQADC will check for the command sequence
coherency until the command sequence ends or until one of the conditions below becomes true:
• The command sequence became non-coherent.
• The CFIFO status changed from the TRIGGERED state.
• The CFIFO had underflow.
Table 19-46. External Buffer Status
Capture
Point at
eQADC
Buffer Status at
External Device
Buffer Status as
Captured by the
eQADC
Used for
NCF
detection on
the eQADC?
(a) EMPTY EMPTY Don’t care
(b) 1 ENTRY EMPTY No
(c) 2 ENTRY 1 ENTRY Yes
SDS
Null Message
Serial Data
Transmitted
1) The CFIFO starts sending commands to an external command buffer when triggered.
2) Execution of a command on the external device takes longer than the time to Assumptions:
complete three serial transmissions.
Command 3 Command 2 Command 1
(a) (b) (c)
Transfer of Command
Sequence Starts External Buffer Status Starts
to be Monitored Here