SYNCHRONOUS SERIAL INTERFACE (SSI)
6 - 94
PORT C
MOTOROLA
6.4.2.3
SSI Status Register (SSISR)
The SSISR is an 8-bit read-only status register used by the DSP to interrogate the status
and serial input flags of the SSI. When the SSISR is read to the internal data bus, the reg-
ister contents occupy the low-order byte of the data bus, and the high-order portion is zero
filled. The status bits are described in the following paragraphs.
6.4.2.3.1
SSISR Serial Input Flag 0 (IF0) Bit 0
The SSI latches data present on the SC0 pin during reception of the first received bit after
frame sync is detected. IF0 is updated with this data when the receive shift register is
transferred into the receive data register. The IF0 bit is enabled only when SCD0 is
cleared and SYN is set, indicating that SC0 is an input and the synchronous mode is se-
lected (see Table 6-5); otherwise, IF0 reads as a zero when it is not enabled. Hardware,
software, SSI individual, and STOP reset clear IF0.
6.4.2.3.2
SSISR Serial Input Flag 1 (IF1) Bit 1
The SSI latches data present on the SC1 pin during reception of the first received bit after
frame sync is detected. The IF1 flag is updated with the data when the receiver shift reg-
ister is transferred into the receive data register. The IF1 bit is enabled only when SCD1
is cleared and SYN is set, indicating that SC1 is an input and the synchronous mode is
selected (see Table 6-5); otherwise, IF1 reads as a zero when it is not enabled. Hardware,
software, SSI individual, and STOP reset clear IF1.
6.4.2.3.3
SSISR Transmit Frame Sync Flag (TFS) Bit 2
When set, TFS indicates that a transmit frame sync occurred in the current time slot. TFS
is set at the start of the first time slot in the frame and cleared during all other time slots.
If word-wide transmit frame sync is selected (FSL0=FSL1), this indicates that the frame
sync was high at least at the beginning of the time slot if external frame sync is selected,
or high throughout the time slot if internal frame sync was selected. If bit-wide transmit
frame sync is selected (FSL0
≠
FSL1), this indicates that the frame sync (either internal or
external) was high during the last Tx clock bit period prior to the current time slot, and that
the frame sync falling edge corresponds to the assertion of the first output data bit, as
shown below.
Time slot #1
Time slot #2
Time slot #3
TFS set here
Bit-Length Fs
Word-Length Fs
Tx shift clock
Time slots
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Freescale Semiconductor, Inc.
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