SN65DSI86: TI SN65DSI86 to eDP Panel (NT156FHM-N41) - No AUX Communication with raspberrypi compute module 4(6.6.y linux kernel)

Hello Bhavin,

Have you probed and observed activity on the AUX channel whenever the transactions are initiated? Please check the waveforms to confirm there is activity over the aux channel.

Please check the datasheet direct and indirect methods for I2C over aux: "8.4.5.3 I2C-Over-AUX" section. Which method are you using? Could you share the script for how you are initiating these steps. 

What mode of DP link training is used for your system? Is the AUX used for link training in the mode you have set up?

Best regards,
Ikram

Not yet, we wanted to confirm our software configuration first. We'll do the probing.

Yes, we are following indirect methods in our test scripts. I am adding the scripts below

EDID_read2.sh
 

#!/bin/bash

I2C_DEV=1
BRIDGE_ADDR=0x2c

# 1. Enable EDID I2C address claim (0x50 with enable bit)
i2cset -y -f $I2C_DEV $BRIDGE_ADDR 0x60 0x61 #0xA1  # 0xA1 = 0b10100001

# 2. Set AUX_ADDR for I2C-over-AUX EDID address (high/mid = 0)
i2cset -y -f $I2C_DEV $BRIDGE_ADDR 0x74 0x00  # AUX_ADDR[19:16]
i2cset -y -f $I2C_DEV $BRIDGE_ADDR 0x75 0x00  # AUX_ADDR[15:8]
i2cset -y -f $I2C_DEV $BRIDGE_ADDR 0x76 0x30 #0x50  # AUX_ADDR[7:0] — EDID address

# 3. Set length to 16 bytes (0x10) — can read full EDID by looping
i2cset -y -f $I2C_DEV $BRIDGE_ADDR 0x77 0x10

# 4. Set command to I2C read (0x09 = I2C-over-AUX Read Request)
#     Format: (CMD << 4) | 0x01 = 0x91
i2cset -y -f $I2C_DEV $BRIDGE_ADDR 0x78 0x91

# 5. Poll until SEND bit (bit 0 of 0x78) is cleared (max timeout 1s)
timeout_ms=1000
elapsed=0
while true; do
    val=$(i2cget -y -f $I2C_DEV $BRIDGE_ADDR 0x78)
    if (( (val & 0x01) == 0 )); then
        break
    fi
    sleep 0.01
    elapsed=$((elapsed + 10))
    if (( elapsed >= timeout_ms )); then
        echo "ERROR: AUX transaction timed out"
        exit 1
    fi
done

# 6. Read back data (AUX_RDATA0 - AUX_RDATA15)
echo "EDID chunk (16 bytes):"
for i in $(seq 0 15); do
    reg=$((0x79 + i))
    byte=$(i2cget -y -f $I2C_DEV $BRIDGE_ADDR $reg)
    printf "%s " "$byte"
done
echo

#!/bin/bash

I2C_DEV=1        # Replace with correct I2C bus
BRIDGE_ADDR=0x2c           # SN65DSI86 I2C address
EDID_ADDR=0x50             # Standard EDID I2C address
NUM_BLOCKS=1               # EDID block count (1 = 128 bytes)
BYTES_PER_BLOCK=128

# Enable I2C Claim1 for EDID address (0x50)
i2cset -y -f $I2C_DEV $BRIDGE_ADDR 0x60 $((EDID_ADDR << 1 | 1)) # 7:1 address + EN bit

# Function to read EDID block (16 bytes at a time)
read_block() {
    local offset=$1
    for ((i=0; i<BYTES_PER_BLOCK; i+=16)); do
        # Set AUX_ADDR[7:0] to EDID address (0x50)
        i2cset -y -f $I2C_DEV $BRIDGE_ADDR 0x77 $EDID_ADDR

        # Set AUX_LENGTH (0x74) to 16
        i2cset -y -f $I2C_DEV $BRIDGE_ADDR 0x74 0x10

        # Set AUX_CMD (0x78) to 0x9 (I2C Read)
        # Set SEND bit (bit 0) to trigger transaction
        i2cset -y -f $I2C_DEV $BRIDGE_ADDR 0x78 0x91

        # Poll until SEND bit clears
        while true; do
            val=$(i2cget -y -f $I2C_DEV $BRIDGE_ADDR 0x78)
            if (( val & 0x01 == 0 )); then
                break
            fi
            printf "Poll until SEND  bit clears"
            sleep 0.01
        done

        # Read 16 bytes from AUX_RDATA0–15 (0x79 to 0x88)
        printf "EDID offset 0x%02X: " $((offset + i))
        for ((j=0; j<16; j++)); do
            byte=$(i2cget -y -f $I2C_DEV $BRIDGE_ADDR $((0x79 + j)))
            printf "%s " "$byte"
        done
        echo
    done
}

# Read full EDID
for ((block=0; block<NUM_BLOCKS; block++)); do
    read_block $((block * BYTES_PER_BLOCK))
done

#!/bin/bash

I2C_DEV=1               # Replace with correct I2C bus (e.g., /dev/i2c-2)
BRIDGE_ADDR=0x2c        # SN65DSI86 I2C address
DPCD_ADDR=(0x00 0x00 0x00)  # DPCD base address is 0x00000

# Write DPCD address to AUX_ADDR registers (0x74, 0x75, 0x76)
i2cset -y -f $I2C_DEV $BRIDGE_ADDR 0x74 ${DPCD_ADDR[0]} b
i2cset -y -f $I2C_DEV $BRIDGE_ADDR 0x75 ${DPCD_ADDR[1]} b
i2cset -y -f $I2C_DEV $BRIDGE_ADDR 0x76 ${DPCD_ADDR[2]} b

# Set AUX length to 16 bytes
i2cset -y -f $I2C_DEV $BRIDGE_ADDR 0x77 0x10 b

# Set AUX_CMD to 0x2 (AUX read)
# Set SEND = 1 to initiate transaction
i2cset -y -f $I2C_DEV $BRIDGE_ADDR 0x78 0x21 b

# Poll until SEND bit clears
for i in {1..100}; do
    val=$(i2cget -y -f $I2C_DEV $BRIDGE_ADDR 0x78)
    if (( (val & 0x01) == 0 )); then
        break
    fi
    sleep 0.01
done

# Check if it timed out
if (( (val & 0x01) != 0 )); then
    echo "Timeout waiting for AUX transaction to complete."
    exit 1
fi

# Read back 16 bytes from AUX_RDATA0–AUX_RDATA15 (0x79–0x88)
echo "DPCD Response:"
for ((i=0; i<16; i++)); do
    reg=$((0x79 + i))
    data=$(i2cget -y -f $I2C_DEV $BRIDGE_ADDR $reg)
    printf "0x%02x " $data
done
echo

#!/bin/bash

I2CBUS=1        # Adjust as per your I2C bus
ADDR=0x2c       # SN65DSI86 I2C address

if [ $# -gt 0 ]; then
	ASSR=1
else
	ASSR=0
fi

echo "ASSR:$ASSR"

raspi-gpio set 20 op dl
sleep 1
raspi-gpio set 20 op dh

# Helper to write a register
w() {
  i2cset -y $I2CBUS $ADDR $1 $2
}

echo "[INFO] Configuring SN65DSI86 for test pattern..."
if [ "$ASSR" -eq 0 ]; then
	echo "configuring Non ASSR"
	w 0xff 0x07
	w 0x16 0x01
	w 0xff 0x00    
fi

# Configure REFCLK 27 MHz
w 0x0a 0x06

# DSI A 4 Lanes
w 0x10 0x20

#DSIA Freq 445MHz
w 0x12 0x59

if [ "$ASSR" -eq 1 ]; then
	w 0x5a 0x05
else
	#Enhance framing NO ASSR
	w 0x5a 0x04
fi

#DP link rate 2.7 Gbps
w 0x94 0x80

# DP Lanes 2, no SSC
w 0x93 0x20

# Enable PLL
w 0x0d 0x01
sleep 0.1

if [ "$ASSR" -eq 1 ]; then
	echo "configuring ASSR"
	w 0x95 0x00
	w 0x64 0x01
	w 0x74 0x00
	w 0x75 0x01
	w 0x76 0x0a
	w 0x77 0x01
	w 0x78 0x81
	sleep 0.10
fi

# Semi auto train
w 0x96 0x0a
sleep 0.2

# Video Parameters
# Horizontal active: 1920
w 0x21 $(( (1920 >> 8) & 0xFF ))
w 0x20 $(( 1920 & 0xFF ))

# Vertical active: 1080
w 0x25 $(( (1080 >> 8) & 0xFF ))
w 0x24 $(( 1080 & 0xFF ))

# Horizontal front porch and back porch
w 0x38 207
w 0x34 48

# HSync width: 32
w 0x2c 32

# vertical front porch and back porch
w 0x3a 71
w 0x36 3

# VSync width: 6
w 0x30 6

# Dp 24 bpp
w 0x5b 0x00

# Color Bar enable
w 0x3c 0x14 #12 = 3 color working

# enhanced framing, ASSR, and Vstream enable
w 0x5a 0x0d

echo "[DONE] Test pattern should now be visible on the panel."

Hi Bhavin,

1. I am not familiar with the driver code that you shared. Could you tell us where you got this?

The software that we have for this device are:

DSI Driver code: https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/bridge/ti-sn65dsi86.c

DSI register calculator: https://e2e.ti.com/support/interface-group/interface/f/interface-forum/945404/faq-sn65dsi86-how-do-i-programming-the-sn65dsi86-registers?tisearch=e2e-sitesearch&keymatch=sn65dsi86#

You can use the calculator tool above as well to input your configurations and generate a script for I2C for initialization.

2. For the I2C over aux steps, please give me 1-2 days to check the "EDID_read" scripts you shared and to match it with the datasheet.

Let me know whether you are able to probe and check for AUX channel activity. Also, you are using the TI EVM to test the DSI86 part right?


Best regards,
Ikram

Hi Bhavin, I will check the script and get back to you.

Thank you,
Ikram

Any update?

Hi Bhavin,

I am sorry for the delay on this. From the "EDID_read2.sh" script, it seems you are setting the AUX_ADDR to 0x30, not 0x50. Is this correct for your application?

Also, you are setting 0x78 to 0x91, instead of 0x41. Can you use the AUX_CMD  = 0x4 for "I2C-Over-Aux Write MOT = 1" option instead?

This previous E2E has a small portion of the code at the end: https://e2e.ti.com/support/interface-group/interface/f/interface-forum/957141/sn65dsi86-q1-i2c-detection-problem

Also, if your system allowed clock stretching, then you can consider using the Direct method to simplify the sequence.

Best regards,
Ikram

Hi Bhavin,

Here's a previous E2E thread about the direct method: https://e2e.ti.com/support/interface-group/interface/f/interface-forum/949449/sn65dsi86-i2c-over-aux-direct-method/3508159

The script syntax is from the Aardvark I2C adapter

Thank you,
Ikram