epaper::hal::SpiBus Concept Reference

Concept for a raw SPI data transfer bus. More...

#include <spi.hpp>

Concept definition

template<typename T>
concept epaper::hal::SpiBus =  requires(T bus, std::uint8_t byte, std::span<const std::byte> data) {
  { bus.transfer(byte) } -> std::same_as<std::uint8_t>;
  { bus.write(data) } -> std::same_as<void>;
}

Detailed Description

Concept for a raw SPI data transfer bus.

Provides full-duplex byte transfer and bulk write capabilities. Does NOT handle Chip Select (CS) or Data/Command (DC) signals - these are managed externally via GPIO pins.

Required Operations:

1. transfer(byte): Full-duplex single-byte exchange (write + simultaneous read)
2. write(data): Bulk write (transmit-only, ignore received data)

Method Signatures:

auto transfer(std::uint8_t byte) -> std::uint8_t;
auto write(std::span<const std::byte> data) -> void;

Semantic Requirements:

• transfer(byte): Transmits byte on MOSI, returns value received on MISO
  • Blocking until transfer complete (typically microseconds)
  • Clock polarity/phase determined by SPI mode (configured externally)
• write(data): Transmits all bytes in data sequentially
  • More efficient than N calls to transfer() (avoids per-byte overhead)
  • Received data discarded (MISO ignored)
  • Blocking until all bytes transmitted

SPI Mode Configuration (not part of concept):

• Mode 0 (CPOL=0, CPHA=0): Clock idle LOW, sample on rising edge
• Mode 1 (CPOL=0, CPHA=1): Clock idle LOW, sample on falling edge
• Mode 2 (CPOL=1, CPHA=0): Clock idle HIGH, sample on falling edge
• Mode 3 (CPOL=1, CPHA=1): Clock idle HIGH, sample on rising edge E-paper displays typically use Mode 0.

Chip Select (CS) Protocol:

cs.write(false);       // Assert CS (active-low)
spi.write(data);       // Transfer data
cs.write(true);        // Deassert CS

Data/Command (DC) Protocol:

dc.write(false);       // DC=0 for command
cs.write(false);
spi.transfer(0x2C);    // Send command byte
cs.write(true);
 
dc.write(true);        // DC=1 for data
cs.write(false);
spi.write(framebuffer);
cs.write(true);

Template Parameters

T	Type to check for SpiBus conformance

See also

DigitalOutput (for CS/DC control), MockSPI