MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
19-58 Freescale Semiconductor
Null Message Format for External Device Operation
Null messages are only transferred through the serial interface to allow results and unsolicited control data,
like the status of the external command buffers, to return when there are no more commands pending to
transfer. Null messages are only transmitted when serial transmissions from the eQADC SSI are enabled
(see ESSIE field in Section 19.3.2.1, “eQADC Module Configuration Register (EQADC_MCR),”), and
when one of the following conditions apply:
1. There are no triggered CFIFOs with commands bound for external command buffers.
2. There are triggered CFIFOs with commands bound for external command buffers but the external
buffers are full. The eQADC detected returning BUSYn fields indicating “Do not send
commands.”
Figure 19-33 illustrates the null message send format. When the eQADC transfers a null message, it
directly shifts out the 26-bit data content inside the Section 19.3.2.2, “eQADC Null Message Send Format
Register (EQADC_NMSFR).” The register must be programmed with the null message send format of the
external device.
Figure 19-34 illustrates the null message receive format. It has the same fields found in a result message
with the exception that the ADC result is not used. Refer to Section , “ Result Message Format for External
Device Operation,” for more information. The MESSAGE_TAG field must be set to the null message tag
(0b1000). The eQADC does not store into an RFIFO any incoming message with a null message tag.
12–15 BUSYn
[0:1]
BUSY status. The BUSY fields indicate if the external device can receive more
commands. Table 19-42 shows how these two bits are encoded. When an external
device cannot accept any more new commands, it must set BUSYn to a value indicating
“Do not send commands” in the returning message. The BUSY fields of values 0b10 and
0b10 can be freely encoded by the external device to allow visibility of the status of the
external command buffers for debug. As an example, they could indicate the number of
entries in an external command buffer.
16–31 ADC_RESULT
[0:15]
ADC RESULT Field. The result data received from the external device or on-chip ADC.
This can be the result of a conversion command, data requested via a read configuration
command, or time stamp value. The ADC_RESULT of any incoming message with a null
message tag will be ignored. When the MESSAGE_TAG is for an RFIFO, the eQADC
extracts the 16-bit ADC_RESULT from the raw message and stores it into the
appropriate RFIFO.
Table 19-42. Command BUFFERn BUSY Status
1
1
After reset, the eQADC always assumes that the external command buffers are full and cannot
receive commands.
BUSYn[0:1] Meaning
0b00 Send available commands—command buffer is empty
0b01 Send available commands
0b10 Send available commands
0b11 Do not send commands
Table 19-41. Result Message Format for External Device Operation (Continued)
Bits Name Description
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