The Serial Peripheral Interface (SPI) is a synchronous, full-duplex, master-slave based communication protocol used for short-distance communication, typically between a microprocessor and peripheral devices . Developed by Motorola in the 1980s, SPI has become a de facto standard in embedded systems due to its high speed and simplicity.

Unlike asynchronous protocols like UART, SPI uses a shared clock signal to synchronize data transfer, allowing for much higher throughput. It is "full-duplex" because data is transmitted (Master to Slave) and received (Slave to Master) simultaneously during every clock cycle .

Footnotes

1. SPI Protocol Overview - General background on SPI signals and master-slave paradigm. ↩

2. SPI Explained - Details on full-duplex and signal lines. ↩

Primary Signal Lines

SPI relies on four primary signal lines to manage communication:

1. SCLK (Serial Clock): Generated by the master to synchronize data bits.
2. MOSI (Master Out Slave In): The line used by the master to send data to the slave.
3. MISO (Master In Slave Out): The line used by the slave to send data back to the master.
4. CS/SS (Chip Select / Slave Select): An active-low signal controlled by the master to "wake up" a specific slave device 2.

Full-Duplex Operation

In SPI, data transfer is essentially a shift-register operation. As the master shifts a bit out on the MOSI line, it simultaneously shifts a bit in from the MISO line. This means that for every byte sent, a byte is received, even if the received data is just "dummy" bytes .

Footnotes

1. SPI Protocol Overview - General background on SPI signals and master-slave paradigm. ↩

2. SPI Explained - Details on full-duplex and signal lines. ↩

3. SPI Demystified - Explanation of simultaneous transmission and reception. ↩

SPI Clock Modes (CPOL and CPHA)

To ensure compatibility between various microprocessors and peripherals, SPI supports four communication modes defined by Clock Polarity (CPOL) and Clock Phase (CPHA) .

• CPOL: Determines if the clock idles at 0 (Low) or 1 (High).
• CPHA: Determines whether data is sampled on the first or second clock edge .

Footnotes

1. SPI Clock Signals and Modes - Deep dive into CPOL, CPHA, and the four SPI modes. ↩ ↩²

Master Timing Analysis

Successful SPI communication requires adhering to specific timing constraints:

• Clock Frequency ($f_{SCLK}$): Must not exceed the maximum frequency of the slowest device on the bus.
• Setup Time ($t_{su}$): The minimum time data must be stable on the line before the clock edge.
• Hold Time ($t_h$): The minimum time data must remain stable after the clock edge .

$f_{max} = \frac{1}{t_{propagation} + t_{setup} + t_{output\_delay}}$

Footnotes

1. Basics of SPI Timing - Timing requirements including setup and hold times. ↩

Interfacing with 8051 Microcontroller

The 8051 can interface with SPI devices in two primary ways:

1. Hardware SPI: Some modern 8051 variants (like those from Silabs or Atmel) include a dedicated SPI peripheral. You simply load the SPDAT register, and the hardware handles the timing.
2. Bit-Banging: On standard 8051s without an SPI port, you manually toggle GPIO pins to mimic the protocol .

Bit-Banging Example (C Code)

1sbit MOSI = P1^0;
2sbit MISO = P1^1;
3sbit SCLK = P1^2;
4
5unsigned char spi_transfer(unsigned char val) {
6    for (int i = 0; i < 8; i++) {
7        MOSI = (val & 0x80) ? 1 : 0; // Set MOSI
8        val <<= 1;                   // Shift next bit
9        SCLK = 1;                    // Clock High
10        if (MISO) val |= 0x01;       // Read MISO
11        SCLK = 0;                    // Clock Low
12    }
13    return val;
14}

Footnotes

1. Bit-Banged SPI for 8051 - Implementation of SPI using GPIO on 8051 microcontrollers. ↩