How to use SPI in LPC2148

This tutorial will cover how to use SPI (Serial Peripheral Interface) with LPC2148. I will only cover the transmit data part, and the receiving will be covered in upcoming tutorials. I am using keil IDE in this tutorial and the process will remain same for other ARM 7 devices too.

SPI uses 4 pins to communicate between the master and the slave device. The pins used are as follows:

SCK -> The Serial Clock
MOSI -> Master Out Slave In is to Write the data to the slave
MISO -> Master In Slave Out is to Read the data from the slave
SSEL -> Slave Select is to Select and Unselect the slave

Registers Used in SPI

Here we will be using the following Registers

S0SPCR -> Control Register to setup the SPI
S0SPSR -> Status Register to check the status of various things
S0SPDR -> Data Register to Write/Receive data
S0SPCCR -> Clock Counter Register to set the frequency for the SPI

SET UP THE SPI

In order to initialize the SPI, we must configure the pins to be used as the SPI pins. This can be done in the PINSEL registers. Below is the picture of the PINSEL0 register of the LPC2148

Since I am only transmitting the data, I don’t need the MISO pin, and hence I only need to setup 3 pins.

To configure the pin P0.4 as the SCK, we need to write a ‘1’ in the 8th bit
To configure the pin P0.6 as the MOSI, we need to write a ‘1‘ in the 12th bit
Pin P0.7 is used to select the Slave, so we will just leave it as the GPIO pin

PINSEL0 |= (1<<8)|(1<<12);  // P0.4 = SCK, P0.6 = MOSI
IODIR0 |= (1<<7);  // P0.7 (SSEL) is output

The steps to set up the SPI are provided in the reference manual of LPC2148

In the above picture, the first 2 steps corresponds to setting up the SPI, and we will follow them. Let’s take a look at the SPI CLOCK COUNTER REGISTER

Clock Counter Register is responsible for setting up the SPI frequency. The value here must be an even number greater than or equal to 8. Since I always keep the PCLK at 60 MHz, the following is a way to setup the desired frequency for the SPI

S0SPCCR = (60000000/freq);

And finally we will check out the SPI CONTROL REGISTER

In the SPI Control Register, I will use the following setup

BitEnable (bit 2) = 0 –> SPI will send and receive 8 bits per transfer
CPHA (bit 3) and CPOL (bit 4) = 0 –> SPI MODE 0, SCK is active HIGH
MSTR (bit 5) = 1 –> SPI operates in Master Mode
LSBF (bit 6) = 0 –> MSB first
SPIE (bit 7) = 0 –> No interrupts
bits 8-11 are not being used (BitEnable = 0)

With the above configuration, the control register setup is shown below

S0SPCR = (1<<5);  // Master mode, 8 bit data, MSB first, CPOL=0 CPHA=0

HOW TO TRANSFER DATA

Before sending the data to the slave, we have to select the slave first. And to do so, we will pull the SSEL pin (P0.7) to LOW

IOCLR0 |= (1<<7);  // P0.7=0 Slave select

In order to transfer the data to the slave, we need to follow the steps 3 and 4 in the picture above

The following is the picture of the SPI STATUS REGISTER

SPIF is the bit 7 of the Status Register, and if this bit is one, it indicates that the data transfer is finished

S0SPDR = data;  // write data 
while (!(S0SPSR & (1<<7)));  //  wait for the SPIF bit to set

Here we will transfer by writing the data into the SPI DATA REGISTER
And then wait for the SPIF (bit 7) in the STATUS REGISTER to set

After the data transfer is finished, we will Unselect the Slave by pulling the SSEL pin HIGH again

IOSET0 |= (1<<7);  // P0.7=1 Slave unselect

The main function

uint8_t data = 0x01;

int main ()
{
	PLL_Init ();
	VPBDIV = 0x01;  // PCLK = CCLK = 60 MHz
	Timer_Init (60);  // PCLK = 60 MHz
	SPI_Init (5000000);  // SPI clock 5MHz, PR = 12 (PR>=8 and should be even)
	
	while (1)
	{
		SPI_Write (data++);
		Delay_ms (1000);  // 1 sec delay
	}
}