Motorola DSP56156 Manual page 250

8.19.1 Normal Operating Mode
In the normal mode, the frame rate divider determines the word transfer rate - one word
is transferred per frame sync, during the frame sync time slot.
8.19.1.1 Normal Mode Transmit
The conditions for data transmission from the SSI are:
1. Transmitter Enabled (TE=1)
2. Frame sync becomes active
When the above conditions occur in normal mode, the next data word will be transferred
from TX to the Transmit shift register, the TDE flag will be set (transmitter empty), and the
transmit interrupt will occur if TIE=1 (Transmit interrupt is enabled). The new data word
will be transmitted immediately.
The transmit data output (STD) is three-stated except during the data transmission period.
The optional frame sync output and clock outputs are not three-stated even if both receiv-
er and transmitter are disabled.
The program must write another word to TX before the next frame sync is received or TUE
will be set and a Transmitter Underrun Error will be generated.
8.19.1.2 Normal Mode Receive
If the receiver is enabled, then each time the frame sync signal is generated (or detected)
a data word will be clocked in. After receiving the data word it will be transferred from the
SSI Receive Shift Register to the Receive Data Register (RX), the RDF flag will be set
(Receiver full), and the Receive Interrupt will occur if it is enabled (RIE=1).
The DSP program has to read the data from RX before a new data word is transferred
from the Receive Shift Register, otherwise the Receiver Overrun error will be set (ROE).
8.19.2 Network Mode
In this mode the SSI can be used in TDM networks and is the typical mode in which the
DSP would interface to a TDM codec network or a network of DSPs. The DSP may be a
master device that controls its own private network or a slave device that is connected to
an existing TDM network and occupies one or more time slots. The distinction of the net-
work mode is that active time slots (data word time) are identified by the transmit and re-
ceive slot mask registers (TSMA0, TSMB0, RSMA0, and RSMB0). For the transmitter,
this allows the option of ignoring a time slot or transmitting data during that time slot. The
receiver is treated in the same manner except that data is always being shifted into the
MOTOROLA SYNCHRONOUS SERIAL INTERFACE (SSI0 and SSI1)
SSI OPERATING MODES
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