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Synchronous Serial Interface (SSI)
27-38
Freescale Semiconductor
Figure 27-29. Normal Mode Timing - External Gated Clock
27.4.1.2
Network Mode
Network mode creates a time division multiplexed (TDM) network, such as a TDM codec network or a
network of DSPs. In continuous clock mode, a frame sync occurs at the beginning of each frame. In this
mode, the frame is divided into more than one time slot. During each time slot, one data word can be
transferred (rather than in the frame sync time slot as in normal mode). Each time slot is then assigned to
an appropriate codec or DSP on the network. The processor can be a master device that controls its own
private network or a slave device connected to an existing TDM network and occupies a few time slots.
The frame rate dividers, controlled by the DC bits, select two to thirty-two time slots per frame. The length
of the frame is determined by:
•
The period of the serial bit clock (PSR, PM bits for internal clock, or the frequency of the external
clock on the SSI_BCLK pin)
•
The number of bits per sample (WL bits)
•
The number of time slots per frame (DC bits)
In network mode, data can be transmitted in any time slot. The distinction of network mode is each time
slot is identified with respect to the frame sync (data word time). This time slot identification allows the
option of transmitting data during the time slot by writing to the SSI_TX registers or ignoring the time slot
as determined by the SSI_TMASK register bits. The receiver is treated in the same manner and received
data is only transferred to the receive data register/FIFO if the corresponding time slot is enabled through
SSI_RMASK.
By using the SSI_TMASK and SSI_RMASK registers, software only has to service the SSI during valid
time slots. This eliminates any overhead associated with unused time slots. Refer to
Transmit Time Slot Mask Register (SSI_TMASK),”
Section 27.3.19, “SSI Receive Time Slot Mask
for more information on the SSI_TMASK and SSI_RMASK registers.
In two channel mode (SSI_CR[TCH] = 1), the second set of transmit and receive FIFOs and data registers
create two separate channels (for example, left and right channels for a stereo codec). These channels are
completely independent with their own set of interrupts and DMA requests identical to the ones available
for the default channel. In this mode, data is transmitted/received in enabled time slots alternately from/to
FIFO 0 and FIFO 1, starting from FIFO 0. The first data word is taken from FIFO 0 and transmitted in the
Tx Data
Rx Data
Gated
SSI_TXD
SSI_RXD
SSI_BCLK
Содержание MCF54455
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