Motorola DSP56156 Manual page 224

1. Write TX data register with data for transmission in the next time slot
2. Write the time slot register to disable transmission in the next time slot
3. Do nothing - transmit underrun will occur at the beginning of the next active time
slot and the previous data will be transmitted
8.19.2.2 Network Mode Receive
The receiver portion of SSI is enabled when RE is set; however, the receive enable will
take place only after detection of a new data frame. After detection of the new data frame,
the first data word will be shifted into the Receive Shift Register during the first active slot.
When the word is completely received, it is transferred to the RX which sets the RDF flag
(Receive Data register full). Setting RDF will cause a receive interrupt to occur if the re-
ceiver interrupt is enabled (RIE=1).
During the second active time slot, the second data word begins shifting in. The DSP pro-
gram must read the data from RX (which clears RDF) before the second data word is com-
pletely received (ready to transfer to RX) or a receive overrun error will occur (ROE gets
set).
If the RE bit is cleared and set again by the DSP programmer, the receiver, after receiving
the current time slot in progress, will be disabled until the next frame sync (first time slot).
This mechanism allows the DSP programmer to ignore data in the last portion of a data
frame.
Note: The optional frame sync output and clock output signals are not affected even if the
transmitter and/or receiver are disabled. TE and RE do not disable bit clock and
frame sync generation.
To summarize, in the Network mode, an interrupt can occur after the reception of each
enabled data word or the programmer can poll the RDF flag. The DSP program response
can be:
1. Read RX and use the data
2. Read RX and ignore the data
3. Do nothing - the receiver overrun exception will occur at the end of the next ac-
tive time slot
8.19.2.3 On-Demand Mode
A divide ratio of one (DC=00000) in the network mode is a special case. This is the only
data driven mode of the SSI and is defined as the on-demand mode. This special case
MOTOROLA SYNCHRONOUS SERIAL INTERFACE (SSI0 and SSI1)
SSI OPERATING MODES
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