Clock polarity and phase (CPOL, CPHA) in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does CPOL stand for in SPI communication?

CPOL stands for Clock Polarity. It defines the idle state of the clock line (SCK) in SPI communication.

beginner

What does CPHA control in SPI communication?

CPHA stands for Clock Phase. It controls when data is sampled and shifted relative to the clock edges.

beginner

How does CPOL = 0 affect the clock line in SPI?

When CPOL = 0, the clock line is low (0) when idle, and the clock pulses go from low to high.

intermediate

Explain the effect of CPHA = 1 on data sampling.

With CPHA = 1, data is sampled on the second clock edge (trailing edge), meaning data changes on the first edge and is read on the second.

intermediate

Why is it important to match CPOL and CPHA settings between SPI master and slave?

Matching CPOL and CPHA ensures both devices sample and shift data at the same clock edges, preventing communication errors.

What is the idle state of the clock line when CPOL = 1?

A Clock line is high

B Clock line is low

C Clock line toggles randomly

D Clock line is disconnected

When CPOL = 1, the clock line stays high when idle.

If CPHA = 0, when is data sampled?

A On the second clock edge

B On the first clock edge

C At the clock idle state

D Data is not sampled

CPHA = 0 means data is sampled on the first clock edge.

Which SPI mode corresponds to CPOL = 0 and CPHA = 0?

A Mode 3

B Mode 1

C Mode 2

D Mode 0

SPI Mode 0 uses CPOL = 0 and CPHA = 0.

What happens if master and slave have different CPOL or CPHA settings?

A Communication works perfectly

B Slave ignores master

C Data may be corrupted or lost

D Only clock speed changes

Different CPOL or CPHA settings cause timing mismatches, leading to data errors.

In SPI, what does CPHA = 1 imply about data changes?

A Data changes on the first clock edge

B Data changes on the second clock edge

C Data changes at idle

D Data does not change

With CPHA = 1, data changes on the first clock edge and is sampled on the second.

Describe how CPOL and CPHA settings affect SPI clock and data timing.

Think about when the clock is high or low and when data is read.

Explain why SPI devices must agree on CPOL and CPHA settings before communication.

Consider what happens if one device reads data too early or too late.