RDK S100 40-pin UART and I2C¶

This handbook follows the recorded course slides, including wiring, commands, measured results, and the scope of verification.

Source slides

Build two minimal links with loopback and a PCF8574T module.

Two buses, two evidence types¶

UART uses transmit-and-receive equality. I2C uses address discovery and data write-back.

UART2¶

Short Pins 8 and 10, then receive the same AA55 bytes.

I2C4¶

Connect Pins 27 and 28 to PCF8574T and detect address 0x20.

Minimal setup¶

No USB-to-TTL adapter, analyzer, or external power supply.

UART and I2C serve different links¶

UART¶

Point-to-point asynchronous communication over TX and RX.

I2C¶

Shared clock and data lines with address-based devices.

Logic level¶

Both S100 40-pin interfaces use 3.3 V logic.

Shared goal¶

Verify the full path from Linux device node to physical wiring.

Understand the SW6 selection¶

I2C5 and UART2 on J24 are selected by the 40 PIN switch. This lesson uses UART2, then keeps it selected while I2C runs on independent I2C4.

Power off before moving SW6
UART2 uses physical Pins 8 and 10
I2C5 uses Pins 3 and 5
I2C4 uses Pins 27 and 28
I2C4 remains available while UART2 is selected

UART2 loopback wiring¶

Use one jumper to connect Pin 8 TXD directly to Pin 10 RXD. Do not connect power or ground for this loopback.

RDK S100 UART2 loopback wiring

Pin 8 TXD ↔ Pin 10 RXD

Enable UART2 in hardware and software¶

UART2 is disabled by default. The verified setup keeps a recovery backup, enables uart2, and disables multiplexed i2c5.

Step 1

Power off and set the SW6 40 PIN switch to UART2.

Step 2

Set uart2 to okay and i2c5 to disabled in the device tree.

Step 3

Reboot and confirm that /dev/ttyS2 exists.

Step 4

Connect Pins 8 and 10, then start the loopback.

Send AA55 and verify the return¶

The measured run uses /dev/ttyS2 at 921600 baud. Transmitted and received bytes match exactly.

Device node¶

ls -l /dev/ttyS2

Confirm the node before testing.

Loopback result¶

TX: AA55
RX: AA55
STATUS: PASS

Compare bytes, not only process startup.

UART loopback evidence¶

Open/dev/ttyS2

Configure921600 8N1

TransmitAA55

ReceiveAA55

AcceptByte match PASS

I2C4 avoids another SW6 change¶

I2C4 is available independently on Pins 27 and 28. The S100 provides 3.3 V power to the module.

Bus¶

/dev/i2c-4

Power¶

Pin 17 supplies 3.3 V and Pin 20 is ground.

Peripheral¶

The measured PCF8574T address is 0x20.

Four-wire PCF8574T connection¶

VCC, ground, SDA, and SCL are all required. The module uses 3.3 V power and 3.3 V logic.

RDK S100 I2C4 PCF8574T wiring

Pin 17→VCC | Pin 20→GND | Pin 27→SDA | Pin 28→SCL

Scan the address before reading¶

A0, A1, and A2 determine the module address, so discovery comes before any hard-coded value.

Scan the bus¶

sudo i2cdetect -y 4

This run finds 0x20; 0x2F is also present on the board.

Official sample¶

sudo python3 /app/40pin_samples/test_i2c.py

Choose i2c-4 and enter the detected address.

I2C read and write evidence¶

EnumerateFind /dev/i2c-4

ScanDetect 0x20

ReadReturn 0xFF

Write backSend a test value

AcceptRead/write PASS

Communication failure checks¶

Verify pin multiplexing and device nodes before changing the wiring.

UART

If ttyS2 is absent, check SW6, the device tree, and reboot.

Loop

If transmit works but receive is empty, inspect Pins 8 and 10.

I2C

If 0x20 is absent, inspect 3.3 V, ground, SDA, and SCL.

Address

Do not mistake the board device at 0x2F for PCF8574T.

complete¶

UART2 loopback and I2C4 peripheral access now have repeatable terminal evidence.

UART2 PASS¶

AA55 transmit and receive bytes match.

I2C4 PASS¶

0x20 is detected, read, and written back.

Next lesson¶

Verify SPI0 full-duplex loopback with one jumper.