Synchronous Serial Interface (SSI)
MCF52277 Reference Manual, Rev. 1
25-8
Freescale Semiconductor
25.3.1
SSI Transmit Data Registers 0 & 1 (SSI_TX0/1)
The SSI_TX0/1 registers store the data to be transmitted by the SSI. For details on data alignment see
Section 25.4.4, “Supported Data Alignment Formats.”
25.3.2
SSI Transmit FIFO 0 & 1 Registers
The SSI transmit FIFO registers are 8x32-bit registers. These registers are not directly accessible. The
transmit shift register (TXSR) receives its values from these FIFO registers. When the transmit interrupt
enable (SSI_IER[TIE]) bit and either of the transmit FIFO empty (SSI_ISR[TFE0, TFE1]) bits are set, an
interrupt is generated when the data level in of the SSI transmit FIFOs falls below the selected threshold.
25.3.3
SSI Transmit Shift Register (TXSR)
TXSR is a 24-bit shift register that contains the data transmitted and is not directly accessible. When a
continuous clock is used, the selected bit clock shifts data out to the SSI_TXD pin when the associated
frame sync is asserted. When a gated clock is used, the selected gated clock shifts data out to the SSI_TXD
port.
The word length control bits (SSI_CCR[WL]) determine the number of bits to shift out of the TXSR before
it is considered empty and can be written to again. The data to be transmitted occupies the most significant
portion of the shift register if SSI_TCR[TXBIT0] is cleared. Otherwise, it occupies the least significant
portion. The unused portion of the register is ignored.
Address: 0xFC0B_C000 (SSI_TX0)
0xFC0B_C004 (SSI_TX1)
Access: User read/write
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
0
R
SSI_TX
W
Reset 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Figure 25-4. SSI Transmit Data Registers (SSI_TX0, SSI_TX1)
Table 25-5. SSI_TX0/1 Field Descriptions
Field
Description
31–0
SSI_TX
SSI transmit data. The SSI_TX0/1 registers are implemented as the first word of their respective Tx FIFOs. Data
written to these registers transfers to the transmit shift register (TXSR), when shifting of the previous data is
complete. If both FIFOs are in use, data alternately transfers from SSI_TX0 and SSI_TX1 to TXSR. SSI_TX1 can
only be used in two-channel mode.
Multiple writes to the SSI_TX registers do not result in the previous data being over-written by the subsequent data.
Instead they are ignored. Protection from over-writing is present irrespective of whether the transmitter is enabled or
not.
Example: If Tx FIFO0 is in use and you write Data1–9 to SSI_TX0, Data9 does not overwrite Data1. Data1–8 are
stored in the FIFO while Data9 is discarded.
Example: If Tx FIFO0 is not in use and you write Data1, Data2 to SSI_TX0, Data2 does not overwrite Data1 and is
discarded.
Note:
Enable SSI (SSI_CR[SSI_EN] = 1) before writing to the SSI transmit data registers
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