What is OutputDrive on the SAI Peripheral?

[APire.1]

On the STM32 SAI peripheral, there's a member of the SAI_InitTypeDef which is:

  uint32_t OutputDrive;         /*!< Specifies when SAI Block outputs are driven.
                                     This parameter can be a value of @ref SAI_Block_Output_Drive
                                     @note This value has to be set before enabling the audio block
                                           but after the audio block configuration. */

However, I can't find any docs on what that means or does anywhere on the documentation or even on Google. Could someone help me understand it?

Thanks

[waclawek.jan · Accepted solution]

Which STM32?

Cube is open source, so you find out what things do simply by looking into it.

Assuming you use e.g. 'F446, look into STM32F4xx_HAL_Driver/Src/stm32f4xx_hal_sai.c to find out what happens with the OutputDrive field:

 hsai->Instance->CR1 |= (hsai->Init.AudioMode | hsai->Init.Protocol |          \

                         hsai->Init.DataSize | hsai->Init.FirstBit |          \

                         ckstr_bits | syncen_bits |                            \

                         hsai->Init.MonoStereoMode | hsai->Init.OutputDrive |  \

                         hsai->Init.NoDivider | (hsai->Init.Mckdiv << 20U));

Okay, so it's planly ORed together with other struct fields into SAIx_CR1.

Then look into the respective header, what are it's values.

#defineSAI_OUTPUTDRIVE_ENABLE ((uint32_t)SAI_xCR1_OUTDRIV)

#defineSAI_OUTPUTDRIVE_DISABLE0x00000000U

Okay, so it's basically just one bit, SAI_xCR1_OUTDRIV. So now look into the RM:

 

So this bit determines, that if SAI is set as Master, whether the respective SAIx_SD_x pin should be output all the time (if OUTDRIV = 1), or if it should go to high-impedance state whenever the respective SAIEN = 0 i.e. the given SAI block is disabled.

JW