Serial Peripheral Interface (SPI) is a high-speed, full-duplex, synchronous serial communication protocol developed by Motorola. It enables microcontrollers and peripheral devices such as shift registers, ADCs, DACs, real-time clocks, memory chips, and display drivers to communicate in a simple and efficient way. The SPI bus typically uses 3 or 4 signal lines: SCLK (clock), MOSI (master-out slave-in), MISO (master-in slave-out), and SS (slave select). This makes it ideal for connecting various peripherals with minimal wiring.
The simplicity of the SPI protocol allows for fast data transfer rates and straightforward software implementation. For example, the AD7888 is a 12-bit, low-power analog-to-digital converter (ADC) that operates on a single supply voltage ranging from 2.7V to 5.25V. It supports up to 125 kSPS (kilo samples per second) and features an internal 2.5V reference, making it suitable for battery-powered applications like PDAs, medical equipment, and mobile communication systems.
In embedded systems, such as those based on the S3C2410 microcontroller from Samsung, the SPI interface is used to connect external peripherals like the AD7888. The S3C2410 is a powerful ARM920T-based processor designed for handheld and embedded applications. It includes two SPI ports, each supporting different modes of operation—polling, interrupt, or DMA. These interfaces allow for efficient data exchange between the microcontroller and peripheral devices.
To implement SPI communication under Linux, device drivers are written to manage the interface. The driver typically includes functions such as initialization, writing to the ADC, reading conversion results, and handling the actual conversion process. Below is a simplified version of such a driver:
```c
void Init_SPI(void) {
rSPPRE0 = 0x32;
rSPCON0 = 0x1e;
rGPHCON |= 0x0400;
rGPHCON &= ~0x0800;
rGPHUP &= ~0x20;
rGPHDAT |= 0x20;
}
ssize_t ad_wr(struct file *file, const char *buf, size_t count, loff_t *offset) {
int i;
unsigned char *dbuf = kmalloc(count * sizeof(unsigned char), GFP_KERNEL);
copy_from_user(dbuf, buf, count);
for (i = 0; i < count; i++)
ADTXdata[i] = dbuf[i];
kfree(dbuf);
return count;
}
ssize_t ad_rd(struct file *file, char *buf, size_t count, loff_t *offset) {
int i;
unsigned char *dbuf = kmalloc(count * sizeof(unsigned char), GFP_KERNEL);
ad_convert();
ad_convert();
for (i = 0; i < count; i++)
dbuf[i] = ADRXdata[i];
copy_to_user(buf, dbuf, count);
kfree(dbuf);
return count;
}
void ad_convert(void) {
rGPHDAT &= ~0x20;
udelay(100000);
spi_tx_data(ADTXdata[0]);
ADRXdata[0] = rSPRDATO;
spi_tx_data(0xff);
ADRXdata[1] = rSPRDATO;
rGPHDAT |= 0x20;
}
void spi_tx_data(unsigned char data) {
spi_poll_done();
rSPTDAT0 = data;
spi_poll_done();
}
static void spi_poll_done(void) {
while (!(rSPSTA0 & 0x01));
}
```
These functions handle the setup of the SPI interface, data transmission, and reception. The `ad_convert()` function performs the actual ADC conversion, while `spi_tx_data()` sends data over the SPI bus. The `spi_poll_done()` function ensures that the SPI transfer is complete before proceeding.
By combining the capabilities of the S3C2410 and the AD7888, developers can build efficient and reliable embedded systems capable of high-speed analog-to-digital conversion. This approach is widely used in applications requiring real-time data acquisition and processing.
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