Motorola DSP56309 User Manual page 202

internal peripherals. During ESSI individual reset, internal DMA accesses to the data
registers of the ESSI are not valid and data read is undefined.
To insure proper operation of the ESSI, use an ESSI individual reset when changing the
ESSI control registers (except for bits TEIE, REIE, TLIE, RLIE, TIE, RIE, TE2, TE1, TE0,
and RE).
Here is an example of initializing the ESSI.
1. Put the ESSI in its individual reset state by clearing the PCR bits.
2. Configure the control registers (CRA, CRB) to set the operating mode. Disable the
transmitters and receiver by clearing the TE[2:0] and RE bits. Set the interrupt
enable bits for the operating mode chosen.
3. Enable the ESSI by setting the PCR bits to activate the input/output signals to be
used.
4. Write initial data to the transmitters that are used during operation. This step is
needed even if DMA is used to service the transmitters.
5. Enable the transmitters and receiver to be used.
Now the ESSI can be serviced by polling, interrupts, or DMA.
Once the ESSI has been enabled (Step 3), operation starts as follows:
¥ For internally generated clock and frame sync, these signals start activity
immediately after the ESSI is enabled.
¥ Data is received by the ESSI after the occurrence of a frame sync signal (either
internally or externally generated) only when the receive enable (RE) bit is set.
¥ Data is transmitted after the occurrence of a frame sync signal (either internally or
externally generated) only when the transmitter enable (TE[2:0]) bit is set.
7.5.3 ESSI Exceptions
The ESSI can generate six different exceptions. They are discussed in the following
paragraphs (ordered from the highest to the lowest exception priority):
1. ESSI receive data with exception status:
Occurs when the receive exception interrupt is enabled, the receive data register
is full, and a receiver overrun error has occurred. This exception sets the ROE bit.
The ROE bit is cleared by first reading the SSISR and then reading RX.
MOTOROLA
Enhanced Synchronous Serial Interface (ESSI)
DSP56309UM/D
Operating Modes
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