QSPI (Quad Serial Peripheral Interface)

📘 QSPI (Quad Serial Peripheral Interface)

1️⃣ Concept

QSPI (Quad SPI) is an extended version of the standard SPI (Serial Peripheral Interface) protocol. It increases data throughput by expanding the data bus from 1 bit to 4 bits.

In standard SPI, data is transferred using MOSI (Master Out Slave In) and MISO (Master In Slave Out), which means 1 bit per clock. In QSPI, these lines are replaced by IO0 ~ IO3, allowing 4-bit parallel data transfer, achieving up to 4× higher throughput than regular SPI without increasing the clock frequency.

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2️⃣ Signal Lines

Unlike standard SPI (which has fixed MOSI/MISO directions), IO0–IO3 are bidirectional and change direction depending on the transfer phase.

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3️⃣ Transfer Phases

Although the exact sequence varies by flash manufacturer, QSPI transactions generally follow three phases:

Example: Reading from QSPI Flash

1. Send 0xEB (Fast Read Quad I/O Command)

2. Send 3-byte address

3. Insert required dummy cycles

4. Receive data in Quad mode

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4️⃣ Example Read Sequence

CS = Low
Command: 0xEB        (Single Line)
Address: 0x00 0x10 0x00    (Quad Line)
Dummy: 8 cycles
Data: [0x12][0x34][0x56]... (Quad Line)
CS = High

Once the command is issued, the flash outputs data using 4 data lines in parallel, increasing read speed.

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5️⃣ Advantages

• High throughput without increasing clock frequency

• Supports XIP (Execute In Place) on some MCUs, enabling direct code execution from flash

• More pin-efficient than parallel flash memory

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6️⃣ Disadvantages

• More complex hardware routing due to timing requirements on 4-bit data lanes

• Command sets differ between vendors (e.g., Winbond, Micron, ISSI)

• Not all MCUs support DMA or memory-mapped mode for QSPI

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7️⃣ Common Commands (Winbond W25Q Series Example)

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8️⃣ Summary

• SPI → 1-bit serial data transfer

• QSPI → 4-bit parallel transfer (up to 4× throughput)

• Used mainly for external flash, firmware storage, and bootloaders

• Some MCUs support XIP mode to execute code directly from QSPI flash

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