RDK S100 40-pin SPI¶

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

Source slides

Verify full-duplex transfer at 12 MHz with one jumper.

SPI0 with the least hardware¶

The S100 transmits two bytes as master and reads them immediately through the MOSI-to-MISO loop.

One jumper¶

Connect Pin 19 MOSI directly to Pin 21 MISO.

12 MHz¶

The official sample sets the bus to 12,000,000 hertz.

Byte acceptance¶

A continuous 0x55 0xAA return is required.

Four core SPI signals¶

SCLK¶

The master clock defines when every data bit is sampled.

MOSI¶

Master output carries the test bytes.

MISO¶

Master input receives the looped-back bytes.

CS¶

Chip select chooses a device; this test opens CS0 through spidev0.0.

SPI0 physical pins on J24¶

The 40-pin digital interface uses 3.3 V logic, and SPI0 supports master mode only.

Pin 19: MOSI
Pin 21: MISO
Pin 23: SCLK
Pin 24: CS0
Pin 26: CS1
This test connects only Pins 19 and 21

SPI0 loopback wiring¶

One jumper connects MOSI to MISO. No power, ground, resistor, or external SPI module is needed.

RDK S100 SPI0 loopback wiring

Pin 19 MOSI ↔ Pin 21 MISO

Confirm spidev0.0¶

The device node connects the Python application to the Linux SPI controller.

List nodes¶

ls -l /dev/spidev*

This run uses /dev/spidev0.0.

Open the controller¶

spi.open(0, 0)
spi.max_speed_hz = 12000000

bus=0 and chip select=0.

Transmitted data returns directly¶

Openspidev0.0

TransmitMOSI: 55 AA

LoopPin 19 to Pin 21

ReceiveMISO: 55 AA

CompareExact match PASS

Run the official SPI sample¶

Enter bus zero and chip select zero. The program transfers one pair of bytes every second.

Command¶

sudo python3 /app/40pin_samples/test_spi.py

Enter 0 and 0 when prompted.

Expected output¶

0x55 0xAA
0x55 0xAA
STATUS: PASS

0x00 0x00 indicates a failed loopback.

Measured SPI0 path¶

Node/dev/spidev0.0

Format8-bit transfer

Speed12,000,000 Hz

Bytes55 AA repeated

ResultLOOPBACK PASS

Four actions in the sample¶

SpiDev¶

Create a Python SPI device object.

open¶

Select bus zero and chip select zero.

xfer2¶

Send a list while receiving another list.

close¶

Release the device handle before exit.

SPI0 acceptance criteria¶

A running process is not enough. The returned data must be checked.

/dev/spidev0.0 exists
Pins 19 and 21 are connected reliably
The bus runs at 12 MHz
The program transmits 0x55 0xAA
The receiver continuously returns 0x55 0xAA
Ctrl+C closes the device handle

Checks for abnormal output¶

The loopback is simple, so faults usually come from the node, jumper, or selected parameters.

Node

If spidev0.0 is absent, first confirm that SPI0 is enabled.

Wire

For 0x00 0x00, power off and inspect Pins 19 and 21.

Select

Use bus zero and chip select zero, not another controller.

Access

For permission errors, use sudo and stop stale device users.

Minimal checks for three buses¶

UART¶

Short TX and RX to verify asynchronous serial transfer.

I2C¶

Scan an addressed peripheral, then read and write.

SPI¶

Short MOSI and MISO for synchronous full-duplex loopback.

complete¶

S100 SPI0 continuously returns 0x55 0xAA at 12 MHz, completing the minimal loopback check.

Minimal wiring¶

Only Pins 19 and 21 are connected.

Clear evidence¶

The terminal output and PASS state are recorded.

Ready to extend¶

The same bus can later connect a real SPI sensor or display.