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Step 7 Programming step 1: SPI UART

The WiFly Shield equips your Arduino with the ability to connect to 802.11b/g wireless networks.
The featured components of the shield are:

• a Roving Network's RN-131G wireless module
• SC16IS750 SPI-to-UART chip. 

Serial Peripheral Interface Bus (or SPI) is a "four wire" serial bus capable of high rates of data transmission. A serial bus allows data to be sent serially (synchronously), i.e. one bit at a time, rather than in parallel (asynchronous)

The Universal asynchronous receiver/transmitter (or UART) is a type of asynchronous receiver/transmitter, a piece of computer hardware that translates data between parallel and serial forms.

1. The PC communicates over UART with the Arduino through pins RX and TX
2. The Arduino communicates over SPI with the SPI-UART chip on the WiFly shield (SC16IS750 SPI-to-UART chip) though pins 10-13 (CS, MOSI, MISO, SCLK respectively)
3. The RN-131G wireless module accesses network and send/receive serial data over UART.

The SPI-to-UART bridge is used to allow for faster transmission speed and to free up the Arduino's UART.

The code below is based on a number of sources, but primarily from this tutorial over at sparkfun:
www.sparkfun.com/commerce/tutorial_info.php 
WiFly Wireless Talking SpeakJet Server

/* Test if the SPI<->UART bridge has been set up correctly by writing a test

   character via SPI and reading it back.

   returns true if success

*/

bool WiFly::TestSPI_UART_Bridge()

{

 // Perform read/write test to check if SPI<->UART bridge is working

 

 // write a character to the scratchpad register.

 WriteByteToRegister(SPR, 0x55);

 

 char data = ReadCharFromWiFly(SPR);

 

 if(data == 0x55)

 {

    return true;

 }

 else

 {

    m_printer->println("Failed to init SPI<->UART chip");

    return false;

 }

}

 

/* A series of register writes to initialize the SC16IS750 SPI-UART bridge chip

   see http://www.tinyclr.com/downloads/Shield/FEZ_Shields_WiFly.cs

*/

void WiFly::SPI_UART_Init(void)

{

 WriteByteToRegister(LCR,0x80); // 0x80 to program baudrate

 WriteByteToRegister(DLL,SPI_Uart_config.DivL); //0x50 = 9600 with Xtal = 12.288MHz

 WriteByteToRegister(DLM,SPI_Uart_config.DivM);

 

 WriteByteToRegister(LCR, 0xBF); // access EFR register

 WriteByteToRegister(EFR, SPI_Uart_config.Flow); // enable enhanced registers

 WriteByteToRegister(LCR, SPI_Uart_config.DataFormat); // 8 data bit, 1 stop bit, no parity

 WriteByteToRegister(FCR, 0x06); // reset TXFIFO, reset RXFIFO, non FIFO mode

 WriteByteToRegister(FCR, 0x01); // enable FIFO mode

}