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Audio Driver HAT on RDK S100

About this lesson: Install the Audio Driver HAT REV2 on the RDK S100, load its drivers, identify the sound card, complete a record → playback loop, and go further with 8-channel capture and playback loopback (the AEC reference signal).

Target hardware: RDK S100

System version: RDK OS (S100)

Related resources: Video walkthrough (in production) / This handbook / Official audio documentation

Adding "listening" and "speaking" to a robot usually starts with a 40pin audio HAT. On the RDK S100, new users rarely get stuck on the commands themselves — they get stuck on three things: the HAT is attached but no sound card appears; it is unclear which device number records and which plays; and the loopback signal that echo cancellation needs is nowhere to be found. This lesson walks the full path in order — recon first, then operate, then verify.

1. Learning Objectives

After completing this lesson, you will be able to:

  1. Describe the role of each chip on the board (2× ES7210 for capture, 1× ES8156 for playback)
  2. Complete the 40pin physical connection and DIP-switch switchover (40 PIN switch left, PCM switch right)
  3. Load the audio drivers with modprobe and confirm card s100snd2 via cat /proc/asound/cards and ls /dev/snd/
  4. Complete a record → playback loop with arecord / aplay, and explain the device numbering: capture on hw:0,0, playback on hw:0,1
  5. Query and adjust playback volume with amixer
  6. Record 8 channels, locate the loopback signal on channels 7–8, and verify it in Audacity
  7. Troubleshoot the three common failure classes: no card, device won't open, no sound

2. Required Materials

Item Qty Notes
RDK S100 1 Flashed and accessible via SSH
Waveshare Audio Driver HAT REV2 1 40pin attachment; 2× ES7210 + 1× ES8156
Headphones or speaker 1 3.5mm interface on the HAT
(Optional) Analog microphones 4 For the mic-array channels

3. Audio System Overview: Reconnaissance First

The S100 audio module is built on the standard ALSA framework, with user-space tools from alsa-lib: arecord, aplay, and amixer. Every recording and playback operation ultimately goes through three layers:

Card                     — one physical audio device = one number
  └─ PCM device          — functional endpoint on the card, playback or capture
       └─ Device node    — a file under /dev/snd/, what programs actually open

For example, pcmC0D0c means "card 0, PCM device 0, capture endpoint" (c = capture, p = playback). Once you understand this naming rule, the command parameters stop being guesswork.

Before touching any configuration, memorize these three commands — every section of this lesson uses them:

# 1. List registered sound cards (the most important one)
cat /proc/asound/cards

# 2. List device nodes in user space
ls /dev/snd/

# 3. List capture / playback devices
arecord -l
aplay -l

Why reconnaissance matters: when you use the -D hw:X,Y parameter, X is the card number from cat /proc/asound/cards, and Y is the device number from arecord -l / aplay -l. Reconnoiter first, then operate — your parameters always have a source.

Device-Number Direction on the S100

Once the HAT is attached and its drivers are loaded, the S100 registers one sound card with two PCM endpoints:

Device node Meaning hw argument Used by
pcmC0D0c card 0 · device 0 · capture hw:0,0 arecord
pcmC0D1p card 0 · device 1 · playback hw:0,1 aplay

Note the direction: capture lives on hw:0,0, playback on hw:0,1. Many tutorials assume "0,0 is playback" — copy that onto the S100 and the device simply won't open. Don't memorize the answer: check the trailing c / p in ls /dev/snd/ every time, and the direction can never be wrong.

4. Meet the Audio HAT: Two Capture Codecs + One Playback Codec

The board used in this lesson is the Waveshare Audio Driver HAT REV2, attached to the S100 through the 40pin header. Its audio chips come in two groups:

Chip Count I2C address Role Channels
ES7210 2 0x40 / 0x42 Capture Up to 8 ch: 4 analog mic inputs + 2 AEC loopback
ES8156 1 0x8 Playback 2 ch, to headphones or speaker

Keep this division of labor in mind and every later observation lines up:

  • Capture goes through the ES7210 pair — 8-channel recording, the mic array, and loopback all live on the capture device;
  • Playback goes through ES8156, 2 channels only, to headphones or a speaker;
  • Four analog microphones pick up sound, and two channels capture "what the speaker actually plays" — that is exactly the reference signal echo-cancellation algorithms need.

5. Hardware Installation: 40pin Connection and the DIP Switches

5.1 Physical Connection

With the board powered off, align the HAT with the S100's 40pin header and press it in firmly.

5.2 DIP Switches: Pin-Function Switchover

Key point: on the S100's 40pin header, the PCM audio pins are multiplexed with the PCIE Wi-Fi module pins. A hardware DIP switch decides which function owns these pins. Before attaching the audio HAT, switch them to the PCM side:

Upper "40 PIN" switch  →  slide LEFT
Lower "PCM" switch     →  slide RIGHT

Move both switches — flipping only one does not complete the switchover.

What happens if you get it wrong? The pins remain connected to the Wi-Fi side. The HAT is physically attached but electrically disconnected — the drivers load fine, yet the card may never appear, or recordings come back all zeros. When "the HAT is attached but nothing happens," check the DIP switches first.

6. Driver Loading and Device Identification

6.1 Load the Drivers

On the S100, the sound card modules ship as kernel modules (ko) and are loaded dynamically:

modprobe hobot_cpudai_super
modprobe snd-soc-es8156
modprobe snd-soc-es7210
modprobe hobot_snd_super_ac_fdx_host

The four commands correspond to four stages of the driver chain: CPU DAI (I2S controller) → playback Codec → capture Codec → sound-card assembly. Only after the last one does the card exist.

6.2 Verify by Reconnaissance

# 1. Card list: s100snd2 appears = drivers loaded
root@ubuntu:~# cat /proc/asound/cards
 0 [s100snd2       ]: s100snd2 - s100snd2
                      s100snd2

# 2. Device nodes: one capture, one playback, one control
root@ubuntu:~# ls /dev/snd/
controlC0  pcmC0D0c  pcmC0D1p  timer

Map these against the direction table in section 3: pcmC0D0c → capture on hw:0,0; pcmC0D1p → playback on hw:0,1.

7. Record and Play: The Simplest Loop

7.1 Recording

# Record 5 seconds, 48kHz / 2ch / 16bit, saved as wav
arecord -Dhw:0,0 -c 2 -r 48000 -f S16_LE -t wav -d 5 test.wav

Parameter by parameter: -D hw:0,0 device (from recon), -c 2 channels, -r 48000 sample rate, -f S16_LE sample format, -t wav file type, -d 5 duration in seconds. Adjust the data format to your needs.

7.2 Playback

aplay -Dhw:0,1 test.wav

aplay parses the wav header to determine the format and feeds the driver, so the playback command does not need to repeat channels or sample rate.

Success criteria: the terminal prints Recording WAVE ... with no errors while recording; you clearly hear the recording during playback.

Why record in 16bit (S16_LE)?

If you record in 24bit (S24_LE), playback on the board with aplay sounds fine, but the file may be pure noise on your computer: ALSA's S24_LE means "24bit stored in a 32bit container" (4 bytes per sample), while the WAV standard's 24bit is 3 bytes per sample. arecord writes a file whose header says 24bit but whose data is 4 bytes per sample, so computer players slice it at 3 bytes and everything misaligns. The board round-trips consistently — that doesn't make the file standard. Recording in 16bit (S16_LE) works everywhere.

7.3 Volume Control (amixer)

# List all controls of the current codec and their values
amixer scontrols
amixer scontents

# Adjust playback volume (the DAC control, for example)
amixer sset 'DAC' 120
amixer sget 'DAC'

As always, confirm the target card first — in multi-card setups use amixer -c 0 ... so you adjust the right one.

8. Advanced: 8-Channel Capture and Loopback

8.1 What Loopback Is and Why You Need It

Loopback captures "the signal on the playback path" as a reference for algorithms such as echo cancellation (AEC). On this board, the loopback signal is mapped to capture channels 7 and 8.

Channels Purpose
ch1–ch4 4 analog microphones
ch7–ch8 Playback loopback reference

8.2 8-Channel Recording

arecord -Dhw:0,0 -c 8 -r 48000 -f S16_LE -t wav -d 5 ./8chn_test.wav --period-size=256 --buffer-size=1024

Watch --period-size: the S100 PDMA driver requires data sizes to be 64-byte aligned, so the period-size value must satisfy this alignment or the transfer will not work properly.

8.3 Verify Loopback with Simultaneous Playback

The loopback records "what the playback path carries," so play while recording, and keep channels, bit depth, and sample rate aligned on the same I2S (this board fixes one I2S lane according to its DIP switches):

# Terminal A: start the 8-channel recording (leave enough time to switch to Terminal B)
arecord -Dhw:0,0 -c 8 -r 48000 -f S16_LE -t wav -d 30 ./8chn_capture.wav --period-size=256 --buffer-size=1024

# Terminal B: simultaneously play a format-aligned file (e.g. a homemade 1kHz sine wave)
aplay -Dhw:0,1 1khz.wav --period-size=1024 --buffer-size=1024

After recording, open 8chn_capture.wav in an audio analysis tool such as Audacity:

  • ch1–ch4 carry the sound picked up by the microphones → the mic path works;
  • ch7–ch8 show the same 1kHz spectrum as the played content → loopback works.

Success criteria: the 8-channel file is generated; speaking into the mics produces waveforms on ch1–ch4, and playing audio produces the matching signal on ch7–ch8.

9. Troubleshooting

Q1: The HAT is attached, but cat /proc/asound/cards shows no s100snd2?

Check three things in order: ① the DIP switches — is the 40 PIN switch left and the PCM switch right; ② did all four modprobe commands run without errors; ③ does dmesg show a codec/I2S probe failure.

Q2: Opening the device node fails?

  • cannot open device ... No such file or directory: the drivers didn't load, or the card/device numbers are wrong — go back to the recon commands;
  • cannot set hw params: Unknown error -22: a parameter (rate/format/channels) exceeds the driver's supported range — use supported values;
  • Opening the node hangs with no log output: ALSA allows each device to be opened only once at a time — check for a previous process that never exited.

Q3: The command succeeds but there is no sound?

Check in order: ① is the target card number right (recon again); ② did you use hw:0,1 for playback (not 0,0); ③ is the volume turned to zero via amixer; ④ does the file format match the device capability.

Q4: Crackling or choppy audio during record/playback?

Most likely an xrun (underrun on playback / overrun on capture: the application can't keep up with the hardware). Try a larger period-size, or write recordings to faster storage to rule out IO bottlenecks; if it happens constantly, investigate the driver or application further.

Q5: What do pcm_read/pcm_write return codes -5 / -32 / -16 mean?

-5: DMA or interrupt trouble (check register configuration and the hardware link); -32: an xrun occurred; -16: the clock dropped or the hardware connection broke during recording (with the S100 acting as slave).

10. Summary

One through-line for the whole lesson:

Hardware in place (40pin aligned + DIP switchover: 40PIN left / PCM right)
   ↓
Drivers loaded (modprobe ×4 → s100snd2 appears in cards)
   ↓ Recon (cards → /dev/snd → confirm D0c capture / D1p playback)
Record & play loop (arecord hw:0,0 → aplay hw:0,1)
   ↓
Advanced loopback (8ch capture, ch7/8 = playback reference, formats aligned)
   ↓
Verify (your ears + Audacity waveforms on every channel)
  • One board, three chips: 2× ES7210 own capture (8ch including loopback), 1× ES8156 owns playback (2ch);
  • Pin multiplexing is the S100's signature trap: the DIP switches decide whether the 40pin pins serve audio or Wi-Fi;
  • Parameters always have a source: period-size must be 64-byte aligned; capture hw:0,0 and playback hw:0,1 are whatever ls /dev/snd/ says.

Extension exercise: record 8 channels while speaking into the mics and playing music through the speaker, then compare the ch1–ch4 and ch7–ch8 waveforms in Audacity to see, concretely, what an AEC reference signal is.

11. References