X40 Hardware Whisperer

I2C devices on the audio boards:

GPIO-controlled chips (no I2C, just enable/reset lines):

• CS5361 ADC, XMOS, Si4730 tuner, output relays

What we need to find on the ribbon cables:

• The I2C bus (SDA + SCL lines going to those three chips)
• The GPIO control lines (enable/reset pins for each chip, relay controls)
• Power rails (3.3V, 5V for logic; 12V+ for amp/relay)

Bus Pirate Quick Start

First connection

1. Plug Bus Pirate into your Mac via USB
2. Find the serial port:
   ls /dev/tty.usbserial*    # or /dev/ttyUSB* on Linux

3. Connect with screen (or minicom):
   screen /dev/tty.usbserial-XXXX 115200

4. Press Enter — you should see:
   HiZ>

5. Type '?' for help, 'i' for version info

The HiZ> prompt means you're in high-impedance mode — all pins are disconnected and safe.

Bus Pirate Pin Colors (v3.6)

I2C Scanning (the first thing to try)

This is the single most useful operation for our project. If we can find the I2C bus on a ribbon cable and scan it, we immediately know which chips are connected.

1. Enter I2C mode:
   HiZ> m        (mode select)
   Select: 4     (I2C)
   Speed: 2      (100kHz — safe default)

2. Enable pull-ups (I2C needs them):
   I2C> P        (uppercase P)

3. Scan for devices:
   I2C> (1)      (address search)

   Expected output for the X40 audio bus:
   Found: 0x49 (0x92)  ← ES9018K2M DAC
   Found: 0x70 (0xE0)  ← SRC4392
   Found: 0x14 (0x28)  ← CS8422 DIR

If you see those three addresses, you've found the main audio I2C bus. That's a huge milestone.

Reading a register

# Read ES9018K2M volume registers
I2C> [0x92 0x0F [0x93 r]    # Write reg addr, restart, read 1 byte
# Should return the left channel volume value

# Read SRC4392 (need page select first)
I2C> [0xE0 0x7F 0x00]       # Page select = 0
I2C> [0xE0 0x03 [0xE1 r]    # Read Port A Control 1

Writing a register (CAREFUL — this changes hardware state)

# Set ES9018K2M volume (safe to try)
I2C> [0x92 0x0F 0x10]       # Left volume = 0x10

# POWER ON sequence (only when you're ready to test)
# This replicates POWER_ON.cfg but the GPIO commands (G W)
# can't be sent via I2C — those are separate physical lines

UART Scanning (for debug ports)

Many embedded boards have unpopulated UART test points. Look for:

• 3 or 4 pads in a row, near the edge of a board
• Labels like TX, RX, GND (or just TP1, TP2, TP3)
• One pin at ~3.3V (TX idles high), one at 0V (GND), one floating (RX)

1. Enter UART mode:
   HiZ> m
   Select: 3     (UART)
   Baud: 9       (115200 — most common)
   Data/Parity/Stop: defaults (8/N/1)

2. Enter transparent bridge mode:
   UART> (1)     (transparent UART bridge)

3. You're now a serial terminal. If the board outputs boot messages
   or a login prompt, you'll see them here.
   Press ~. to exit (tilde then period)

Step-by-Step Probing Procedure

Session 1: Identify the I2C bus

Equipment: Multimeter + Bus Pirate Target: The ribbon cable connecting the SMP8670 board to the audio processing board (#4) or audio output/DAC board (#5)

Step 1: With everything powered OFF and unplugged, disconnect the target
        ribbon cable from the SMP8670 board (leave it connected to the
        audio board end).

Step 2: Count the pins on the ribbon cable connector. Write it down.

Step 3: Power on the unit (the audio boards should get power from the
        power board, not the SMP8670 board — verify this).

Step 4: With the multimeter, measure each pin against the chassis ground:
        - Note which pins show 0V (ground)
        - Note which pins show ~3.3V (likely signal lines with pull-ups,
          or power rails)
        - Note which pins show ~5V or ~12V (power rails)
        - Note which pins show ~1.5V (could be I2S clock averaging)
        - Note floating/unstable pins (inputs waiting for data)

Step 5: Power off. Connect Bus Pirate:
        - GND to a ground pin you identified
        - MOSI (grey) to a 3.3V pin that might be SDA
        - CLK (purple) to another 3.3V pin that might be SCL
        (I2C lines idle at 3.3V because of pull-up resistors)

Step 6: Power on. Enter I2C mode and scan.
        If you find 0x49, 0x70, or 0x14 — you've got the right pins.
        If not, try a different pair of 3.3V pins and scan again.

Session 2: Map the GPIO control lines

Equipment: Multimeter only (no Bus Pirate needed) Target: The same ribbon cable, focusing on the non-I2C pins

The GPIO lines are simple digital outputs from the SMP8670. With the SMP8670 board disconnected, these pins will be floating. We can identify them by:

1. Noting which pins on the ribbon cable go to the enable/reset pins of known chips on the audio board (trace visually or with continuity mode)
2. Looking at the chip datasheets — the reset pin on the SRC4392, ES9018K2M, CS5361, XMOS will each connect to one GPIO line

Session 3: Display interface

Target: The ribbon cable to the display controller board (#8)

The original X40 used DirectFB for display rendering, which means the SMP8670 had a framebuffer output. This could be:

• Parallel RGB (many pins — 8-24 data + sync signals)
• SPI (4-6 pins — if it's a small SPI LCD)
• LVDS (differential pairs — unlikely for 5")

Pin count is the biggest clue. An SPI display uses 4-6 wires. A parallel RGB display uses 20+.

Wiring the Pico 2

Once we've identified all the signals, the Pico 2 wiring is straightforward:

Pico 2 Pin    →  X40 Signal          Notes
─────────────────────────────────────────────
GP0 (I2C0 SDA) → Audio I2C SDA       Main audio bus
GP1 (I2C0 SCL) → Audio I2C SCL       Main audio bus
GP2            → GPIO reg 69          Audio power on/off
GP3            → GPIO reg 62          8670/XMOS select
GP4            → GPIO reg 63          Audio switch 2
GP5            → GPIO reg 64          Audio switch 1
GP6            → GPIO reg 52          XMOS reset
GP7            → GPIO reg 55          ADC CS5361 reset
GP8            → GPIO reg 56          DAC ES9018 reset
GP9            → GPIO reg 57          SRC4392 reset
GP10           → GPIO reg 54          Tuner reset
GP11           → GPIO reg 27          XLR output enable
GP12           → GPIO reg 29          RCA output enable
GP13           → GPIO reg 53          Headphone output enable
GP14           → GPIO reg 68          Coaxial digital out
GP15           → GPIO reg 70          TOSLINK digital out
GP16           → Left encoder A       PIO quadrature decode
GP17           → Left encoder B
GP18           → Left encoder switch
GP19           → Right encoder A      PIO quadrature decode
GP20           → Right encoder B
GP21           → Right encoder switch
GP22-26        → Nav buttons          GPIO with interrupts
GP27 (ADC)     → Sense pin (optional) Monitor a voltage rail
GND            → Common ground        Multiple ground connections
VBUS (5V)      → From USB             Powered by host computer

This uses 22 of the Pico 2's 30 GPIO pins, leaving 8 spare for the display interface or future expansion.

Troubleshooting

Bus Pirate won't connect:

• Check the serial port name — it changes if you unplug/replug
• Make sure no other program has the port open
• Try screen /dev/tty.usbserial-XXXX 115200 (Mac) or screen /dev/ttyUSB0 115200 (Linux)

I2C scan finds nothing:

• Check that the audio board is actually powered (measure 3.3V rail)
• Verify you have GND connected
• Try enabling pull-ups: P command
• Try swapping SDA and SCL (you might have them backwards)
• Try 50kHz speed instead of 100kHz: some devices are slow

I2C scan finds wrong addresses:

• Make sure you're on the right ribbon cable
• Some I2C devices have configurable addresses via hardware pins
• The 7-bit vs 8-bit address convention is confusing: Bus Pirate shows 7-bit, the .cfg files use 8-bit (shifted left by 1)

Multimeter shows unexpected voltages:

• 1.8V on a signal line: the board might use 1.8V logic (some modern chips do)
• 0V on everything: the board isn't getting power — check the power board connection
• Voltage fluctuating rapidly: that's a clock or data line — normal for active signals