SD Card Interfacing With LPC1768- (Part 15/21)

Fig. 1: SD Card Interfacing With LPC1768 Prototype

SD card is consisting of two basic semiconductor sections, a ‘memory core’ and a ‘SD card controller’. The ‘memory core’ is the flash memory region where the actual data of the file is saved. When we format the SD card a file system will be written into this region. Hence this is the region where the file system exists. The ‘SD card controller’ helps to communicate the ‘memory core’ with the external devices like microcontrollers. It can respond to certain set of standard SD commands and read or write data from the memory core in for the external device.

For more details refer the below article.

Interfacing SD Card with AVR Microcontroller.

In this project the SPI bus is used for the serial interface. The Circuit diagram shows how to interface more than one SD card with a microcontroller. In this kind of interfacing the microcontroller is called the ‘Host’ and the SD card is called the ‘Slave’. The microcontroller initiates all the data transfers. The clock is also controlled by the microcontroller. The microcontroller is free to choose between the SD cards by asserting the respective CS (Chip Select) pin.

The data is transmitted from the microcontroller to the SD card using the MOSI (Master Output Slave Input) channel and the data is transferred by the SD card to the microcontroller using the MISO (Master Input Slave Output) channel.

The pin out of a SD card for the SPI interfacing mode is shown in the following figure.

Fig. 2: Pin Name And Description of SD Card for SPI Interfacing Mode

Fig. 3: Pin Diagram Of Sd Card

The circuit diagram for the serial interfacing of a SD card with the microcontroller including the level converter is shown in the circuit diagram tab.

LPC1768 SPI BLOCK:

POWERING THE SPI:

PCON register bits description:

Bit	Symbol	Description	Reset value
0	–	Reserved.	NA
1	PCTIM0	Timer/Counter 0 power/clock control bit.	1
2	PCTIM1	Timer/Counter 1 power/clock control bit.	1
3	PCUART0	UART0 power/clock control bit.	1
4	PCUART1	UART1 power/clock control bit.	1
5	–	Reserved.	NA
6	PCPWM1	PWM1 power/clock control bit.	1
7	PCI2C0	The I2C0 interface power/clock control bit.	1
8	PCSPI	The SPI interface power/clock control bit.	1
9	PCRTC	The RTC power/clock control bit.	1
10	PCSSP1	The SSP 1 interface power/clock control bit.	1
11	–	Reserved.	NA
12	PCADC	A/D converter (ADC) power/clock control bit.	0
13	PCCAN1	CAN Controller 1 power/clock control bit.	0
14	PCCAN2	CAN Controller 2 power/clock control bit.	0
15	PCGPIO	Power/clock control bit for IOCON, GPIO, and GPIO interrupts.	1
16	PCRIT	Repetitive Interrupt Timer power/clock control bit.	0
17	PCMCPWM	Motor Control PWM	0
18	PCQEI	Quadrature Encoder Interface power/clock control bit.	0
19	PCI2C1	The I2C1 interface power/clock control bit.	1
20	–	Reserved.	NA
21	PCSSP0	The SSP0 interface power/clock control bit.	1
22	PCTIM2	Timer 2 power/clock control bit.	0
23	PCTIM3	Timer 3 power/clock control bit.	0
24	PCUART2	UART 2 power/clock control bit.	0
25	PCUART3	UART 3 power/clock control bit.	0
26	PCI2C2	I2C interface 2 power/clock control bit.	1
27	PCI2S	I2S interface power/clock control bit.	0
28	–	Reserved.	NA
29	PCGPDMA	GPDMA function power/clock control bit.	0
30	PCENET	Ethernet block power/clock control bit.	0
31	PCUSB	USB interface power/clock control bit.	0

Fig. 4: Bits Description Of PCON Register For POWERING SPI In LPC1768

Following reset the PCSPI (Power Clock Control Bit to SPI) bit is set and the ADC is enabled.

SELECTING CLOCK FOR SPI:

By assuming that the main clock for the LPC1768 has been programmed. Each LPC1768 peripheral including the ADC has a clock derived from the main clock as illustrated.

Peripheral Clock Divider

As shown in the below block the frequency of the peripheral clock is determined by two bits in the PCLKSEL registers. ADC is included in PCLKSEL0. With the NXP LPC1768 the peripheral clocks are always active. Following RESET PCLKSEL registers are cleared setting the peripheral clock frequency to CCLK/4 to all the peripherals. The user will have a choice of frequency which can be determined by two bits for each peripherals among the peripheral clock selection registers PCLKSEL0 and PCLKSEL1. By default at reset all values are 00 ie CCLK/4.

 

00	PCLK_peripheral = CCLK/4
01	PCLK_peripheral = CCLK
10	PCLK_peripheral = CCLK/2
11	PCLK_peripheral = CCLK/8, except for CAN1, CAN2, and CAN filtering when “11” selects = CCLK/6.

Fig. 5: Bit Combination Of PCLKSEL Registers For Clock Selection

 

Bit	Symbol	Description
1:0	PCLK_WDT	Peripheral clock selection for WDT.
3:2	PCLK_TIMER0	Peripheral clock selection for TIMER0.
5:4	PCLK_TIMER1	Peripheral clock selection for TIMER1.
7:6	PCLK_UART0	Peripheral clock selection for UART0.
9:8	PCLK_UART1	Peripheral clock selection for UART1.
11:10	–	Reserved.
13:12	PCLK_PWM1	Peripheral clock selection for PWM1.
15:14	PCLK_I2C0	Peripheral clock selection for I2C0.
17:16	PCLK_SPI	Peripheral clock selection for SPI.
19:18	–	Reserved.
21:20	PCLK_SSP1	Peripheral clock selection for SSP1.
23:22	PCLK_DAC	Peripheral clock selection for DAC.
25:24	PCLK_ADC	Peripheral clock selection for ADC.
27:26	PCLK_CAN1	Peripheral clock selection for CAN1.[1]
29:28	PCLK_CAN2	Peripheral clock selection for CAN2.[1]
31:30	PCLK_ACF	Peripheral clock selection for CAN acceptance filtering.[1]

Fig. 6: Bit Values And Description Of PCLKSEL Register In LPC1768 ‘s SPI 

Selecting the SPI function to GPIO:

The below block diagram shows the SPI input pins multiplexed with other GPIO pins. The SPI pin can be enabled by configuring the corresponding PINSEL register to select SPI function.
When the SPI function is selected for that pin in the Pin Select register, other Digital signals are disconnected from the SPI input pins.

SPI PINS	Port Pin	Pin Functions	Associated PINSEL Register
SCK	P0.15	0-GPIO, 1-TXD1, 2-SCK0, 3-SCK	30,31 bits of PINSEL0
SSEL	P0.16	0-GPIO, 1-RXD1, 2-SSEL0, 3-SSEL	0,1 bits of PINSEL1
MISO	P0.17	0-GPIO, 1-CTS1, 2-MISO0, 3-MISO	2,3 bits of PINSEL1
MOSI	P0.18	0-GPIO, 1-DCD1, 2-MOSI0, 3-MOSI	4,5 bits of PINSEL1

Fig. 7: SPI Input Pins Multiplexed With GPIO Pins In LPC1768

 

SPI REGISTERS:

The registers associated with LPC1768 ADC are

• SPCR -> SPI Control Register : used to configure SPI

• SPSR -> SPI status Register : Check the status.

• SPDR – > SPI Data Register : contains received data or data to be transmitted

• SPCCR -> SPI Clock Counter Register : used to control master SCK frequency
   

SPI REGISTER CONFIGURATION:

SPCR (SPI Control Register)

Bit	Symbol	Description	Reset value
1: 0	–	Reserved, user software should not write ones to reserved bits.	NA
2	BIT ENABLE	0-> The SPI controller sends and receives 8 bits of data per transfer. 1-> The SPI controller sends and receives the number of bits selected by bits 11:8	0
3	CPHA	0-> Data is sampled on the first clock edge of SCK. A transfer starts and ends with activation and deactivation of the SSEL signal. 1-> Data is sampled on the second clock edge of the SCK. A transfer starts with the first clock edge, and ends with the last sampling edge when the SSEL signal is active.	0
4	CPOL	Clock polarity control. 0-> SCK is active high. 1-> SCK is active low.	0
5	MSTR	Master mode select. 0-> The SPI operates in Slave mode. 1-> The SPI operates in Master mode.	0
6	LSBF	LSB First controls which direction each byte is shifted when transferred. 0-> SPI data is transferred MSB (bit 7) first. 0-> SPI data is transferred LSB (bit 0) first.	0
7	SPIE	Serial peripheral interrupt enable. 0-> SPI interrupts are inhibited. 1-> A hardware interrupt is generated each time the SPIF or MODF bits are activated.	0
11:8	BITS	When bit 2 of this register is 1, this field controls the number of bits per transfer: 1000-> 8 bits per transfer 1001-> 9 bits per transfer 1010-> 10 bits per transfer 1011-> 11 bits per transfer 1100-> 12 bits per transfer 1101-> 13 bits per transfer 1110-> 14 bits per transfer 1111-> 15 bits per transfer 0000-> 16 bits per transfer	0000
31:12	–	Reserved, user software should not write ones to reserved bits.	NA

Fig. 8: Bit Configuration of SPCR Control Register in SPI

SPSR (SPI status Register)

The S0SPSR register controls the operation of SPI0 as per the configuration bits setting.

Bit	Symbol	Description	Reset value
2:0	–	Reserved, user software should not write ones to reserved bits.	NA
3	ABRT	Slave abort. When 1, this bit indicates that a slave abort has occurred. This bit is cleared by reading this register.	0
4	MODF	Mode fault. when 1, this bit indicates that a Mode fault error has occurred. This bit is cleared by reading this register, then writing the SPI0 control register.	0
5	ROVR	Read overrun. When 1, this bit indicates that a read overrun has occurred. This bit is cleared by reading this register.	0
6	WCOL	Write collision. When 1, this bit indicates that a write collision has occurred. This bit is cleared by reading this register, then accessing the SPI Data Register.	0
7	SPIF	SPI transfer complete flag. When 1, this bit indicates when a SPI data transfer is complete. When a master, this bit is set at the end of the last cycle of the transfer. When a slave, this bit is set on the last data sampling edge of the SCK. This bit is cleared by first reading this register, then accessing the SPI Data Register. Note: this is not the SPI interrupt flag. This flag is found in the SPINT register.	0
31:8	–	Reserved, user software should not write ones to reserved bits.	NA

Fig. 9: Bit Value Of SPSR Status Register For Operation Control In SPI

SPDR (SPI Data Register )

Bit	Symbol	Description	Reset value
7:0	DataLow	SPI Bi-directional data port.	0x00
15:8	DataHigh	If bit 2 of the SPCR is 1 and bits 11:8 are other than 1000, some or all of these bits contain the additional transmit and receive bits. When less than 16 bits are selected, the more significant among these bits read as zeroes.	0x00
31:16	–	Reserved, user software should not write ones to reserved bits.	NA

Fig. 10: Bit value of SPDR Data Register in SPI

SPCCR (SPI Clock Counter Register )

Bit	Symbol	Description	Reset value
7:0	Counter	SPI0 Clock counter setting.	0x00
31:8	–	Reserved, user software should not write ones to reserved bits.	NA

Fig. 11: Bit Value Of SPCCR Clock Counter Register In SPI

Create a project using Keil uvision4 for LPC1768 Microcontroller:

In this section, we will start creating a project in Keil MDK we have already installed Keil µVision and Co-MDK Plug-in + CoLinkEx Drivers required for the CoLinkEx programming adapter. You can start by downloading the project files and kick start your practical experiment.

Code.rar

Code Description:

First we have to initialize and mount the SD card, this can be done by following functions.

status = disk_initialize(0);

result = f_mount(0,&drive);

In this example creating and writing to a text file will be explained. First the file has to be opened. The command for this is result = f_open(&file, “test.txt”, FA_CREATE_NEW);

There are different modes of opening a file. The modes are:

FA_READ

FA_WRITE

FA_READ and FA_WRITE are self-explaining. There are some more commands. Those are explained on the bottom of the tutorial. A correct command to open a file called “test.txt” for reading is:

result = f_open(&file1, “test.txt”, FA_READ );

After a file is opened it is possible to read and write from that file. The command to read data from the file is: result = f_read(File, Buffer, bytes to read, bytes done); File is the opened file. Buffer is a buffer where all the data will be placed in. Buff is a variable declared earlier for this. Bytes to read is the amount of bytes that will be read from the file. This can be a variable or just a number. Bytes done is a variable that will contain the number of bytes that have been read successfully.

To write to a file it has to be opened with FA_WRITE. The command to write to a file is: result = f_write(File, Data to be written, Number of byte to write, Bytes done); File is the opened file. Data to be written can be the Buff variable or just a string of text. Number of bytes to write can be a variable or just a number. Bytes done is a variable that will contain the number of bytes that have been read successfully. A correct command to write a string of text to a file is:

result = f_write(&file1, “Hello world “, 11, &s2);

When you are done with reading/writing to a file ALWAYS close the file! Not doing this can result in hard to find bugs. The command to close a file is result = f_close(file1);