VHDL Macro: DC97

From JavierValcarce.Eu

VHDL Macro

component dc97 
  port (
    n_reset      : in  std_logic; --active-low
    clk          : in  std_logic;
    -- ac97 interface signals
    ac97_sdata_o : out std_logic;
    ac97_sdata_i : in  std_logic;
    ac97_sync    : out std_logic;
    ac97_bitclk  : in  std_logic;
    ac97_reset   : out std_logic;
    --
    ready        : out std_logic;
    L_o          : in  std_logic_vector(15 downto 00);
    R_o          : in  std_logic_vector(15 downto 00);
    L_i          : out std_logic_vector(15 downto 00);
    R_i          : out std_logic_vector(15 downto 00);
    cmd_addr     : in  std_logic_vector(07 downto 00);
    cmd_data     : in  std_logic_vector(15 downto 00));
end component;

This VHDL macro is a simple AC97 audio controller^[1] for play/rec stereo 16-bit 48kHz. It has an extremely simple interface. It doesn't have neither FIFO queues to store audio samples nor DMA signals but an interrupt mechanism and, therefore, it must be serviced in real-time. After a ready signal pulse, you have aprox. 10us to transfer audio data, if not, the opportunity is lost and you must wait to the next pulse.

• sampling frequency is fixed at 48kHz
• 16-bit resolution
• play/rec stereo (L, R channel)
• this macro in intended to let you do stereo audio experiments moving data in/out with a simple FSM controller (or PicoBlaze) without the burden of a complete SoC bus interface, but, you can implement FIFO queues and, e.g., a Wishbone bus interface on top on this macro if necessary (it's not that difficult).

Ports And Usage

The macro has the following ports:

The AC97 codec has internal hardware registers that store information like volume, gain, record source select, etc. Refer to the datasheet of your codec to more information. For example: register 0x1A selects record source (MIC is 0x0000, LineIn is 0x0404). You can use dc97cmd block (included in source file) which is a simple FSM that periodically sends a standard configuration data (select input and volume). Connect dc97cmd to dc97 cmd_addr and cmd_data ports.

Interface with PicoBlaze

If you want to use PicoBlaze to process audio (and why not?) the recommended way to do this is to use interrupts. Maintain the ready interrupt signal with a FF like this:

-- interrupt retainer flip-flop 
process (n_reset, clk)
begin
    if n_reset = '0' then
        interrupt <= '0';
    elsif rising_edge(clk) then            
        if interrupt_ack = '1' then
            interrupt <= '0';
        elsif ready = '1' then
            interrupt <= '1';            
        end if;
    end if;
end process;

The ISR (Interrupt Service Routine) could store each audio sample in a buffer with a capacity of N samples prior to process them. The technique is to maintain two buffers to communicate ISR and main program and interchange its contents when required (after N interrupts).

With PicoBlaze running @27MHz and with 48kHz sampling frequency, you have (27M / 2) / 48000 = 281 instructions per sample. It is not a huge DSP capacity but enough to relay audio samples to HW co-processors or to put them in simple frame formats (multimedia containers like OGG).

Take in mind that, if you plan to do a relatively complex relaying/processing, PicoBlaze will be smaller than the equivalent FSM (Finite State Machine). Do not underestimate the power and compactness (< 100 slices) of PicoBlaze!

Download

• dc97.vhd This file contains two components: dc97 (the AC97 controller) and dc97cmd (a small FSM to to setup codec's registers). The latter block is not strictly required, you can setup registers by software, for example.

References

• http://web.mit.edu/6.111/www/f2008/handouts/labs/lab4.html
• http://www.analog.com/en/audiovideo-products/audio-codecs/ad1981b/products/product.html

1. ↑ Digital Controller' 97 (DC97) in the nomenclature used in the standard document