Audio Core

The Audio Core is a circuit which simplifies communication between the Nios II processor and the Audio CODEC responsible for converting sound into digital values (enCOding) and vice versa (DECoding). It is essentially 2 pairs of FIFOs (so 4 in total) each 128 entries deep with each pair responsible for sending/receiving samples to/from the LINEOUT/MIC. They are pairs because there are two different channels, left and right, allowing for stereo sound (stereo sound means the left and right speakers can play different sounds, while mono sound means they both play the same). To better understand how analog input is converted to digital bits read the following documentation understanding sound .
See the image below for a diagram of the Audio Core and its FIFOs.

Audio controller FIFOs
DeviceAudio Core
Configuration32-bit signed integer samples, two-channel, 48 KHz sampling rate, 128 sample FIFO
Address Base0xFF203040
Address Map
baseR/WControl Register for clearing FIFOs and interrupt control/status. See Note 1
base+4R/WFifospace Register tells you how much space is in each of the 4 FIFOs. See Note 2.
base+8R/WLeft Data, reads read from the MIC FIFO, writes write to the LINEOUT FIFO.
base+12R/WRight Data, reads read from the MIC FIFO, writes write to the LINEOUT FIFO.
TriggeredRead Interrupts - when either left/right FIFO is 75% or more filled.
Write Interrupts - when either left/right FIFO is 75% or more empty
IRQ Line6
EnableIn Control Register set bits 1 (WE) and 0 (RE) for write and read interrupts respectively.
AcknowledgeInterrupts will keep reoccurring until FIFOs are below/above the 75% mark. See Note 3
Hardware SetupPlug the microphone into the MIC port, and the speakers into the LINEOUT audio port. The LINEIN port is played back on LINEOUT, but is otherwise unused (not recorded).
Reference Audio Core Datasheet
WM8731 Audio CODEC Datasheet


  1. The Control Register uses only the least significant 6 bits as follows:
    0RERead Interrupt enable
    1WEWrite Interrupt enable
    2CRClear both left and right read FIFOs
    3CWClear both left and right write FIFOs
    8RIIndicates that a read interrupt is pending
    9WIIndicates that a write interrupt is pending
  2. The Fifospace Register stores how much space is in each of the 4 FIFOs. The register is 32-bits wide and since each FIFO has only 128 entries, the Fifospace register can store all 4 of the 8-bit values.
    7:0RA RCRead data Available in Right Channel
    15:8RA LCRead data Available in Left Channel
    23:16WS RCWrite Space in Right Channel
    31:24WS LCWrite Space in Left Channel
  3. There is no Interrupt Acknowledge, you simply need to make sure that for every interrupt you appropriately pop/push something to/from the violating FIFO. As long as your Interrupt Service Routine is faster than the 48KHz of the CODEC, the processor will equilibrate with the CODEC and interrupt only once with every new sample. The Nios processor runs about 2000 times faster than the CODEC's sampling rate, so avoid executing more than 2000 instructions in your ISR per sample processed.
  4. The LINEIN port is echoed back out LINEOUT in hardware, but is not recorded. This configuration can be changed by editing the DE1-SoC Computer and rebuilding it.

IMPORTANT: You must always write to both left and right FIFOs. If either FIFO is empty, nothing is sent to the Audio CODEC.

Assembly Example: Echo program - connects mic to speakers, polling

  ldwio r3,4(r2)      # Read fifospace register 
  andi  r3,r3,0xff    # Extract # of samples in Input Right Channel FIFO 
  beq   r3,r0,main    # If no samples in FIFO, go back to start 
  ldwio r3,8(r2)
  stwio r3,8(r2)      # Echo to left channel 
  ldwio r3,12(r2)
  stwio r3,12(r2)     # Echo to right channel 
  br main

C Example: Echo program - connects mic to speakers, polling

// We're lazy: Read only from right channel.
int main()
	unsigned int fifospace;
	volatile int * audio_ptr = (int *) 0xFF203040; // audio port
	while (1)
		fifospace = *(audio_ptr+1); // read the audio port fifospace register
		if ((fifospace & 0x000000FF) > 0 &&		// Available sample right
			(fifospace & 0x00FF0000) > 0 &&		// Available write space right
			(fifospace & 0xFF000000) > 0)		// Available write space left
			int sample = *(audio_ptr + 3);	// read right channel only
			*(audio_ptr + 2) = sample;		// Write to both channels
			*(audio_ptr + 3) = sample;