SPI with external devices (sensors, displays) in Embedded C - Step-by-Step Execution

Concept Flow - SPI with external devices (sensors, displays)

Start SPI Communication

↓

Set Chip Select LOW

↓

Send Data Byte

↓

Wait for Transmission Complete

↓

Read Received Data

↓

Set Chip Select HIGH

↓

End SPI Communication

This flow shows how SPI communication starts by selecting the device, sending and receiving data, then ending communication by deselecting the device.

Execution Sample

Embedded C

SPI_CS_LOW();
SPI_Transmit(0xA5);
while(!SPI_Transmission_Complete());
uint8_t data = SPI_Receive();
SPI_CS_HIGH();

This code selects the SPI device, sends a byte 0xA5, waits for transmission to complete, reads the received byte, then deselects the device.

Execution Table

Step	Action	SPI_CS State	Data Sent	Transmission Complete?	Data Received
1	Set Chip Select LOW	LOW	-	No	-
2	Send Data 0xA5	LOW	0xA5	No	-
3	Wait for Transmission Complete	LOW	0xA5	No	-
4	Transmission Complete	LOW	0xA5	Yes	-
5	Read Received Data	LOW	0xA5	Yes	0x3C
6	Set Chip Select HIGH	HIGH	0xA5	Yes	0x3C

💡 Chip Select set HIGH ends SPI communication with the device.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	After Step 5	Final
SPI_CS	HIGH	LOW	LOW	LOW	LOW	HIGH
Data Sent	-	-	0xA5	0xA5	0xA5	0xA5
Transmission Complete	No	No	No	Yes	Yes	Yes
Data Received	-	-	-	-	0x3C	0x3C

Key Moments - 3 Insights

Why do we set Chip Select LOW before sending data?

Setting Chip Select LOW tells the external device to listen and communicate. Without this, the device ignores SPI signals. See Step 1 in execution_table.

Why do we wait for transmission to complete before reading data?

SPI is synchronous; data is exchanged simultaneously. Waiting ensures the sent byte is fully transmitted and the received byte is ready. See Steps 3 and 4.

What happens if we don't set Chip Select HIGH after communication?

The device stays selected and may misinterpret further SPI signals. Setting it HIGH ends communication properly. See Step 6.

Visual Quiz - 3 Questions

Look at the execution_table, what is the SPI_CS state after Step 3?

A Undefined

B HIGH

C LOW

D Toggling

💡 Hint

Check the SPI_CS State column at Step 3 in execution_table.

At which step does the transmission complete become 'Yes'?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the Transmission Complete? column in execution_table.

If we skip setting Chip Select HIGH at the end, what is the likely effect?

A Communication ends normally

B Device remains selected and may misbehave

C Data is lost immediately

D SPI bus resets automatically

💡 Hint

Refer to key_moments explanation about Chip Select behavior.

Concept Snapshot

SPI communication with devices:
1. Set Chip Select LOW to start.
2. Send and receive data byte.
3. Wait for transmission complete.
4. Read received data.
5. Set Chip Select HIGH to end communication.
Always control Chip Select to target the right device.

Full Transcript

This visual trace shows SPI communication steps with an external device like a sensor or display. First, the Chip Select line is set LOW to activate the device. Then a data byte (0xA5) is sent. The program waits until the transmission completes, ensuring data is fully sent and received. After that, the received data byte (0x3C) is read from the SPI buffer. Finally, Chip Select is set HIGH to end communication. Variables like SPI_CS state, data sent, transmission status, and data received are tracked step-by-step. Key points include why Chip Select must be controlled and why waiting for transmission completion is necessary. The quiz checks understanding of SPI_CS state, transmission timing, and Chip Select importance.