Serial Peripheral Interface (SPI) is a synchronous serial communication protocol developed by Motorola, enabling microprocessors to interface with peripheral devices such as TTL shift registers, A/D and D/A converters, real-time clocks (RTC), memory modules, and display drivers like LCDs and LEDs. This protocol allows for efficient data exchange using only 3 to 4 signal lines, making it ideal for expanding I/O capabilities in embedded systems.
The SPI bus offers strong hardware support and simple software implementation, making it a popular choice for applications requiring high-speed data transfer. For instance, serial A/D converters are known for their compact design and reliable operation. When integrated with ARM chips, which are commonly used in handheld and embedded systems, the combination of SPI technology and these converters ensures effective data acquisition and processing.
The AD7888 is a high-performance, low-power 12-bit analog-to-digital converter from Analog Devices. It operates on a single supply voltage ranging from 2.7 V to 5.25 V and can achieve a maximum throughput of 125 kSPS. The device features an internal 2.5 V reference and supports an external reference between 1.2 V and VDD. Its CMOS process ensures minimal power consumption—only 2 mW during normal operation and 3 µW in power-down mode. The AD7888 supports multiple power management modes and is compatible with various serial interfaces such as SPI, QSPI, MICROWIRE, and DSP. It is widely used in battery-powered systems like PDAs, medical devices, and mobile communications, as well as in instrumentation and high-speed modems. The chip is available in 16-pin SOIC and TSSOP packages.
Figure 2 illustrates the detailed timing diagram of the AD7888's serial interface, showing how the conversion process works. The chip select (CS) signal initiates data transfer and conversion. Once CS falls, the input signal is sampled after 1.5 clock cycles, referred to as tACQ. The entire conversion process takes 14.5 clock cycles, known as tCONVERT. In total, 16 clock cycles are required to complete the conversion and retrieve the result. After the rising edge of CS, the bus returns to a high-impedance state. If CS remains low, the system prepares for the next conversion. To ensure accurate sampling, the channel selection must be pre-programmed into the control register before the conversion begins.
The S3C2410 is a 16/32-bit RISC microprocessor from Samsung, based on the ARM920T core. It features a five-stage pipeline, Harvard architecture, and a maximum operating frequency of 203 MHz. The processor is optimized for low power consumption and high performance, making it suitable for embedded systems and portable devices. It includes a variety of built-in peripherals, such as an LCD controller, NAND flash boot loader, SDRAM controller, UARTs, DMA channels, PWM timers, I/O ports, RTC, ADC, touch screen interface, I2C, IIS, USB host/device ports, SD/MMC interface, two SPI ports, and two PLLs.
Each SPI port on the S3C2410 has separate shift registers for transmitting and receiving data. Data transmission is synchronized, and the clock frequency can be configured via control registers. If only sending data, the received data is ignored; if only receiving, dummy data "0xff" must be sent. The SPI interface uses four signals: serial clock (SCK), master-in slave-out (MISO), master-out slave-in (MOSI), and active-low chip select (nSS). Key features include compatibility with SPI protocol version 2.11, 8-bit shift registers for both transmit and receive, an 8-bit frequency setting register, and support for polling, interrupt, and DMA modes.
To implement SPI communication between the S3C2410 and the AD7888 under Linux, a driver was developed. The driver includes several key functions:
1. **Init_SPI()**: Initializes the SPI interface.
2. **ad_wr()**: Writes the selected channel for A/D conversion.
3. **ad_rd()**: Reads the result of the A/D conversion.
4. **ad_convert()**: Executes the actual A/D conversion.
5. **spi_tx_data()**: Sends data over the SPI bus.
6. **spi_poll_done()**: Polls the SPI status until the transfer is complete.
These functions work together to enable seamless communication between the microcontroller and the A/D converter, ensuring accurate and efficient data acquisition in embedded systems.
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