SSI as i2s master with 16 bit audio

[FredrikMoller]

It it possible to configure the SSI in i2s master mode while playing or recording 16 bit audio?

According to the reference manual for i.MX6, the "word length is fixed to 32 in I2S Master mode". Is there any way to work around this limitation? Or is it simply impossible to use master mode if one wants 16 bit?

[FredrikMoller · Accepted solution]

The following workaround seems to work:  Set SSIx_SCR.I2S_MODE to 0 (normal mode) instead of 1 (I2S master mode). This has been tested in loopback mode as well as with an external codec. Driver was initialised with the following register settings:

SCR = 0x00001119
   SSIEN = 0x1
   TE = 0x0
   RE = 0x0
   NET = 0x1
   SYN = 0x1
   I2S_MODE = 0x0        <<<------ Normal mode instead of i2s master mode
   SYS_CLK_EN = 0x0
   TCH_EN = 0x1
   CLK_IST = 0x0
   TFR_CLK_DIS = 0x0
   RFR_CLK_DIS = 0x0
   SYNC_TX_FS = 0x1
SISR = 0x00003003
SIER = 0x00280000
STCR = 0x000003ED
   TEFS = 0x1
   TFSL = 0x0
   TFSI = 0x1
   TSCKP = 0x1
   TSHFD = 0x0
   TXDIR = 0x1
   TFDIR = 0x1
   TFEN0 = 0x1
   TFEN1 = 0x1
   TXBIT0 = 0x1
SRCR = 0x0000078D
   REFS = 0x1
   RFSL = 0x0
   RFSI = 0x1
   RSCKP = 0x1
   RSHFD = 0x0
   RXDIR = 0x0
   RFDIR = 0x0
   RFEN0 = 0x1
   RFEN1 = 0x1
   RXBIT0 = 0x1
   RXEXT = 0x1
STCCR = 0x0000E116
   PM7_PM0 = 0x16
   DC4_DC0 = 0x1
   WL3_WL0 = 0x7
   PSR = 0x0
   DIV2 = 0x0
SRCCR = 0x00040000
SFCSR = 0x00880088
   TFWM0 = 0x8
   RFWM0 = 0x8
   TFCNT0 = 0x0
   RFCNT0 = 0x0
   TFWM1 = 0x8
   RFWM1 = 0x8
   TFCNT1 = 0x0
   RFCNT1 = 0x0
SACNT = 0x00000000
SACADD = 0x00000000
SACDAT = 0x00000000
SATAG = 0x00000000
STMSK = 0xFFFFFFFC
SRMSK = 0xFFFFFFFC

[igorpadykov]

Hi Frederik

"word length is fixed to 32 in I2S Master mode" is SSI hardware limitation,

unfortunately there is no way to work around this limitation.

Best regards
igor
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[FredrikMoller]

Hi Igor, and thanks for the reply!

Since posting this question, I've tried a potential workaround. Instead of setting SSIx_SCR.I2S_MODE to 1 (I2S master mode) I set it to 0 (normal mode). All other register settings are the same as described in the i.MX 6 reference manual chapter 61.8.1.4 "I2S Mode" when configuring I2s master mode, including those bits that would be internally overridden had I2S master mode been selected in SSIx_SCR.I2S_MODE.

The samples generated are now 16 bit instead of 32, as seen by oscilloscope. I've also done loopback tests where the received samples are the same as the sent samples. I havent't tried to communicate with a real audio codec yet.

It would be great if you could look at this workaround and see if there is any flaw in it, or if it is safe.

Best regards,

Fredrik