How to configure the SAI as slave on the I2S bus

[bimorqt]

Hello everyone,

I am trying to replicate this approach to integrate STM32MP1 with an external audio codec.

The issue I am facing is that the sub-block SAI-A is generating the bitclock (SCK), while I need it to receive it from the external audio codec. I am sniffing the I2S bus and whenever the audio is played, I can see the SCK being generated by SAI-A.

Could you please provide details on how to set the SAI in slave mode, so that the SCK is not generated?

Thanks in advance!

[PatrickF]

Hi @bimorqt​ 

I'm not expert, but according to wiki, it seems 'bitclock-master;' is required in the codec device Tree.

https://wiki.st.com/stm32mpu/wiki/SAI_device_tree_configuration#Sharing_the_codec_interface_between_two_SAIs

Could you share your Device Tree ?

Some posts which might help in this area:

https://community.st.com/s/question/0D53W00000HKwSZSA1

https://community.st.com/s/question/0D53W00001SeXRGSA3

Regards,

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[bimorqt]

Hello @PatrickF​ 

thank you for the quick response. I tried to replicate the devicetree provided here. Here are the most salient parts related to audio of my devicetree:

sound {
	compatible = "audio-graph-card";
	label = "AUDIO;
	routing =
			"HP_OUT", "Digital Input Mux",
			"LINE_OUT", "Digital Input Mux",
			"AIFOUT", "Capture Mux";
	dais = <&sai2a_port &sai2b_port>;
	status = "okay";
};
 
&i2c1{
	pinctrl-names = "default", "sleep";
	pinctrl-0 = <&i2c1_pins_mx>;
	pinctrl-1 = <&i2c1_sleep_pins_mx>;
	status = "okay";
 
	/* USER CODE BEGIN i2c1 */
	#address-cells = <1>;
	#size-cells = <0>;
	i2c-scl-rising-time-ns = <100>;
	i2c-scl-falling-time-ns = <7>;
	/delete-property/dmas;
	/delete-property/dma-names;
 
	sgtl5000: codec@0a { 
		compatible = "fsl,sgtl5000"; 
		reg = <0x0a>;
		#sound-dai-cells = <0>; 
		clocks = <&sai2a>; /* The codec is a consumer of SAI2A master clock */
		clock-names = "MCLK";
		VDDA-supply = <&v3v3>; 
		VDDIO-supply = <&v1v8>; 
 
		sgtl5000_port: port {
			#address-cells = <1>;
			#size-cells = <0>;
			sgtl5000_tx_endpoint: endpoint@0 {
				reg = <0>;
				remote-endpoint = <&sai2a_endpoint>;
				frame-master; // if I comment these two lines, the audio playback works
				bitclock-master;
			};
 
			sgtl5000_rx_endpoint: endpoint@1 {
				reg = <1>;
				remote-endpoint = <&sai2b_endpoint>;
				frame-master; // if I comment these two lines, the audio playback works
				bitclock-master;
			};
		};
	};
 
	/* USER CODE END i2c1 */
};
 
&sai2{
	pinctrl-names = "default", "sleep";
	pinctrl-0 = <&sai2a_pins_mx>, <&sai2b_pins_mx>;
	pinctrl-1 = <&sai2a_sleep_pins_mx>, <&sai2b_sleep_pins_mx>;
	status = "okay";
 
	/* USER CODE BEGIN sai2 */
	clocks = <&rcc SAI2>, <&rcc PLL3_Q>, <&rcc PLL3_R>;
	clock-names = "pclk", "x8k", "x11k";
	/* USER CODE END sai2 */
 
	sai2a:audio-controller@4400b004{	// Playback DAI
		status = "okay";
 
		/* USER CODE BEGIN sai2a */
		compatible = "st,stm32-sai-sub-a";
		#clock-cells = <0>;
		dma-names = "tx";
		clocks = <&rcc SAI2_K>;
		clock-names = "sai_ck";
 
		sai2a_port: port {	
			sai2a_endpoint: endpoint {
				format = "i2s";
				remote-endpoint = <&sgtl5000_tx_endpoint>;
				mclk-fs = <256>;
			};
		};
		/* USER CODE END sai2a */
	};
 
	sai2b:audio-controller@4400b024{	// Capture DAI
		status = "okay";
 
		/* USER CODE BEGIN sai2b */
		compatible = "st,stm32-sai-sub-b";
		dma-names = "rx";
		st,sync = <&sai2a 2>;
		clocks = <&rcc SAI2_K>, <&sai2a>;
		clock-names = "sai_ck", "MCLK";
 
		sai2b_port: port {
			sai2b_endpoint: endpoint {
				remote-endpoint = <&sgtl5000_rx_endpoint>;
				format = "i2s";
				mclk-fs = <256>;
			};
		};
		/* USER CODE END sai2b */
	};
};

If I leave lines 40-41 uncommented, both the SAI-A and the codec generate SCK and audio playback does not work. If I leave lines 40-41 commented and lines 48-49 uncommented, the audio playback is heavily disturbed by a high-frequency noise (the source media file is basically unhearable). A working solution for audio playback is to comment lines 40-41 and 48-49, but in these conditions the audio capture does not work, because SCK is not present on the I2S bus.