Lecture 12: SPI and SD cards
Contents
1. IDAT QOOOOOOSO0 Ye 9C 54 BB CD Ge 2C 49 Ve A5 FY F4 CY CC DD Z3 xeTse In wiCl Oooo006o0 zz 33 6F FD 74 75 FS OF DO OO 67 40 99 A1 90 32 3oytu o B ghHT 2 00000070 23 E4 FB BF O3 iF 80 OB 90 98 41 03 DD 5D 55 F7 ate a Ue ddA DE CC 0G 77 357 53 DS 33 8B 4G S CY 37 B1 OA 97 Pl w72803 74 QOOOOOOSO0 S 73 53 FS A3 ET VO 26 3F FC AF FF De B OF CD 885 c e usyoiol 0OOOOOCAO 8 61 D2 32 99 99 CC D2 42 99 4E 16 04 88 BC 2D aOzz mMI OBmN A OOOOoOBO DB DB ES B2 2C 6D AZ 1E BF FD 9C 11 73 22 10 i19 Dat me dels oooooocOo EE 56 8E 82 49 82 D4 Bi CF 9C 73 48 A6 E4 D BB iV2 1 Ot esd al 0O0OOCOODO 84 42 64 E6 BO AE AA 36 CD 39 E3 20 03 41 47 B5 Bde S 5198 icy QOOOOOOEO C5 DS AD 56 31 58 GE Fa ES AF SF B4 FO 75 DS 4C X18Z S aul Offset a Overwrite A Figure 11 Contents of png file displayed using HxD editor The program in this example found as the entire uVision project lab5_sd_dbg inside the downloadable archive of lab 5 simply 1 initializes the SPI and the SD card and 2 reads data from the SD card starting at address 0x00040000 and then prints byte by byte to the putty terminal of the host PC This relatively simple set up is very useful for debugging purposes and for investigating micro SD cards which working with is kind of painful as a lot of times SD cards do not behave as theoretically described on sdcard org or for example in SanDisk s ambiguous documentat2. To receive this message your program should continuously toggle the SD CLK signal and observe the MISO line for data while keeping the MOSI line high and the CS line low Your program can detect the message because every message begins with a 0 bit and when the SD card sends no data it keeps the MISO line high Note that the response to each command is sent by the card a few SD CLK cycles later If the expected response is not received within 16 clock cycles after sending the reset command the reset command has to be sent again Following a successful reset test if your system can successfully communicate with the SD card by sending a different command For example send one of the following commands while keeping the CS at value 0 1 Command CMD8 01 001000 00000000 00000000 00000001 10101010 0000111 1 5 This command is only available in the latest cards compatible with SD card Specifications version 2 0 For most older cards this command should fail and cause the SD card to respond with a message that this command is illegal 11 Command CMD58 01 111010 00000000 00000000 00000000 00000000 0111010 1 This command requests the contents of the operating conditions register for the connected card A response to these commands consists of 40 bits see Fig 7 where the first 8 bits are identical to the basic 8 bit response while the remaining 32 bits contain specific information about the SD card Although the actual contents of the
3. Your program will have to toggle the SD CLK signal and maintain the MOSI line high while waiting for a response The length of a response message varies depending on the command Most of the commands get a response mostly in the form of 8 bit messages with two exceptions where the response consists of 40 bits To ensure the proper operation of the SD card the SD CLK signal should have a frequency in the range of 100 to 400 kHz To communicate with the SD card your program has to place the SD card into the SPI mode To do this set the MOSI and CS lines to logic value 1 and toggle SD CLK for at least 74 cycles After the 74 cycles or more have occurred your program should set the CS line to 0 and send the command CMD0 01 000000 00000000 00000000 00000000 00000000 1001010 1 This is the reset command which puts the SD card into the SPI mode if executed when the CS line is low The SD card will respond to the reset command by sending a basic 8 bit response on the MISO line The structure of this response is shown in Fig 6 The first bit 1s always a 0 while the other bits specify any errors that may have occured when processing the last message If the command you sent was successfully received then you will receive the message 00000001 76543210 DER in idle state erase reset illegal command CRC error erase sequence error address error parameter error Figure 6 Format of a basic 8 bit Response to every command in SPI mode
4. SD driver utilizes the SPI interface to communicate with the SD Card Fortunately it is not necessary to write all of this software One can use of the FatFs generic file system or use licensed ones but for pay like the one from Keil This open source package provides most of the components required including a generic SD driver that 1s relatively easily modified to utilize with any typical SPI driver To understand how an application interacts with FatFs consider the example derived from the FatFs sample distribution illustrated in Fig 10 credit http homes soic indiana edu geobrown c335 from where portions of this description have been adopted too This example fragment assumes it is communicating with an SD card formatted with a fat file system which contains file in the root directory called MESSAGE TXT The program reads this file and creates another called HELLO TXT Notice the use of relatively standard file system commands f mount O0 amp Fatfis Register volume work area Eprinti n0pen an existing file Maessage txrt n rc f open amp Fil MESSAGE TET FA_READ if rc xprintf XnType the fila content n for f Read a chunk of file rc f raad EFil Buff sizeof Buff amp br if rc br break Error or end of file for 1 0 1 br 1 Type the data y nyputchar Buff 1 n 1f rc die rc xprintf nClose the file n re f closa EFi1 if rc di
5. only high capacity gt 4G cards Please take a while and read the following very popular webpage that describes the use of MMC and SDC cards http elm chan org docs mmc mmce e html A lot of the concepts described in the aforementioned webpage apply also to working with micro SD cards which we ll use in the examples studied later on in this lecture A block diagram of the SD card is shown in Fig 4 It consists of a 9 pin interface a card controller a memory interface and a memory core The 9 pin interface allows the exchange of data between a connected system and the card controller The controller can read write data from to the memory core using the memory core interface In addition several internal registers store the state of the card The controller responds to two types of user requests control and data Control requests set up the operation of the controller and allow access to the SD card registers Data requests are used to either read data from or write data to the memory core L b I ig DAT DAT3CMD CLK DATO DATI Interface Driver OCRBLO Ll CID 127 0 RCA 15 0 DSE 15 0 CSD 127 0 SCE 63 0 Memory Core Interface Figure 4 Block diagram of an SD card B Communication with SD cards Communication with an SD card can be done in one of two modes the SD mode or the SPI mode By default the SD card operates in the SD mode However we ll work with the SPI mode and communicate with it using the SPI
6. protocol Communication with the SD card is performed by sending commands to it and receiving responses from it A valid SD card command consists of 48 bits as shown in Fig 5 The leftmost two bits are the start bits which we set to 01 They are followed by a 6 bit command number and a 32 bit argument where additional information may be provided Next there are 7 bits containing a Cyclic Redundancy Check CRC code followed by a single stop bit 1 47 46 45 40 39 57 10 and Number Argument Figure 5 Format of the 48 bit command for an SD card The CRC code is used by the SD card to verify the integrity of a command it receives By default the SD card ignores the CRC bits for most commands except CMDS unless a user requests that CRC bits be checked upon receiving every message sending a command to the SD card is performed in serial fashion By default the MOSI line is set to 1 to indicate that no message is being sent The process of sending a message begins with placing its most significant bit on the MOSI line and then toggling the SD CLK signal from 0 to 1 and then back from 1 to 0 Then the second bit is placed on the MOSI line and again the SD CLK signal is toggled twice Repeating this procedure for each bit allows the SD card to receive a complete command Once the SD card receives a command it will begin processing it To respond to a command the SD card requires the SD CLK signal to toggle for at least 8 cycles
7. related references How to use MMC SDC cards very popular reference http elm chan org docs mmc mmce e html Modified version of the above http users ece utexas edu valvano EE345M view12 SPI SDC pdf Lab manual http homes soic indiana edu geobrown c335 Application note http alumni cs ucr edu amitra sdcard Additional sdcard_appnote_foust pdf https www sdcard org home SDCARD Physical Layer Simplified Specification https www sdcard org downloads pls simplified specs archive partl 101 pdf Toshiba SD Card specification http embdev net attachment 39390 TOSHIBA SD Card Specification pdf MultiMediaCard MMC http en wikipedia org wiki MultiMediaCard Secure Digital SD Card http en wikipedia org wiki Secure Digital card FatFs Generic FAT File System Module http elm chan org fsw ff 00index_e html 3 PNG and BMP formats http en wikipedia org wiki Portable Network Graphics http en wikipedia org wiki BMP file format 12
8. Lecture 12 SPI and SD cards EE 379 Embedded Systems and Applications Electrical Engineering Department University at Buffalo Last update Cristinel Ababei March 2013 1 Objective The objective of this lecture is to learn about Serial Peripheral Interface SPI and micro SD memory cards 2 SPIIntroduction Serial Peripheral Interface SPI communication was used to connect devices such as printers cameras scanners etc to a desktop computer but it has largely been replaced by USB SPI is still utilized as a communication means for some applications using displays memory cards sensors etc SPI runs using a master slave set up and can run in full duplex mode 1 e signals can be transmitted between the master and the slave simultaneously When multiple slaves are present SPI requires no addressing to differentiate between these slaves There is no standard communication protocol for SPI SPI is used to control peripheral devices and has some advantages over I2C Because of its simplicity and generality it is being incorporated in various peripheral ICs The number of signals of SPI three or four wires is larger than I2C s two wires but the transfer rate can rise up to 20 Mbps or higher depends on device s ability 5 50 times faster than I2C Therefore often it is used in applications ADC DAC or communication between ICs that require high data transfer rates The SPI communication method is illustrated in Fig 1 below The m
9. aster IC and the slave IC are tied with three signal lines SCLK Serial Clock MISO Master In Slave Out and MOSI Master Out Slave In The contents of both 8 bit shift registers are exchanged with the shift clock driven by master IC An additional fourth signal SS Slave Select is utilized to synchronize the start of packet or byte boundary and to facilitate working with multiple slave devices simultaneously Most slave ICs utilize different pin names e g DI DO SCK and CS to the SPI interface For one way transfer devices such as DAC and single channel ADC either of data lines may be omitted The data bits are shifted in MSB first Master Slave MSE MISO Shift Register ADC DAC Comm Memory etc Shift Register j MCU DSP etc Figure 1 SPI communication method When additional slaves are attached to the SPI bus they are attached in parallel and an individual SS signal must be connected from the master to each of the slaves as shown in Fig 2 The data output of the slave IC 1 is enabled when the corresponding SS signal is set the data output 1s disconnected from the MISO line when the slave device 1s deselected Figure 2 Single master multiple slaves configuration In SPI data shift and data latch are done on opposite clock edges Consequently the four different operation modes as a result of the combination of clock polarity and clock phase that the master IC can configure its SPI interfac
10. e rc Eprintf nCreate a new file hello trxt Xn rc f open amp Fil HELLU TXT F WRITE F CREATE ALWATYT3 if rc die rc rprintf inWrita a text data Hello world Xn re f_write amp Fil Hello world r n 14 amp bw 1f rc die rc rprintf AZu bytes written n bw Figure 10 FatFs example 4 Example 1 Reading from a micro SD card 1GB raw data from fixed physical addresses In this example I simply read raw data from a fixed physical address on the micro SD memory card The SD card has only one file which is a small png image alfaromeo png with size 320x240 pixels Using the free HxD hexadecimal editor I first found that the physical address where the file 1s stored inside the memory is 0x00040000 This is a neat editor which also shows the contents of the file as in figure below ET HHD M BUFFALO ee embedded systems slabsslab5 sd usb alfaromeo pnul mi x E File Edit Search View Analvsis Extras Window x B alfaromeo png DOrrserrh 00 Bi Hz 03 DT B5 06 BI Oe BS DA HB Bb D OE OF pDupgagagagdg 89 50 4E 47 OD O4 14 Of OO OO OO OD 49 45 44 5z PNG ae ar ae eae IHDE noonogoaigd OO 00 ai 40 ao ag OO FO 8 2 OO OO OO FE 4F z 8 85 po ddz 3c OO OO OO 06 62 4B 47 44 FF OO FF OO FF HEGD Y Y 9 00000030 BD AT 93 00 OO OO 09 70 48 59 73 00 OO OF C4 OO amp S pHYs 00000040 OO OE C4 O1 95 2B OF 1B OO OO 20 OO 49 44 41 54
11. e are shown in Fig 3 below SPI Mode Timing Diagram Mode 0 Positive Pulse Latch then Shift CPHA 0 CPOL 0 Mode 1 Positive Pulse Shift then Latch Mode 2 Negative Pulse Latch then Shift Mode 3 Negative Pulse Shift then Latch Figure 3 The four different operation modes of SPI There is a lot of online information on SPI You should search and read some for more details Good starting points are the references suggested in 1 from where most of the above material has been adopted Also read Chapter 17 of the LPC17xx User Manual for details on the SPI interface available on the LPC1768 MCU that we use in this course 3 MMC and SDC Cards A Background The Secure Digital Memory Card SDC is the de facto standard memory card for mobile devices The SDC was developed as upper compatible to Multi Media Card MMC SDC compliant equipment can also use MMCs in most cases These cards have basically a flash memory array and a micro controller inside The flash memory controls erasing reading writing error controls and wearleveling are completed inside the memory card The data is transferred between the memory card and the host controller as data blocks in units of 512 bytes therefore these cards can be seen as generic hard disk drives from the view point of upper level layers The currently defined file system for the memory card is FAT12 16 with FDISK petitioning rule The FAT32 1s defined for
12. get it to work unless you use a known card and an already written program for that card 4 Capacity C Card Capacity Figure 8 Flowchart of a generic SD card initialization routine in SPI mode C FAT File system basics Microcontrollers are relatively memory constrained e g 512K bytes flash which limits the ability to store large quantities of data either as inputs to or outputs from an embedded program For example in a game application it might be desirable to access sound and graphic files or in a data logging application to store extended amounts of data In addition accessing the contents of the flash requires a special interface and software In such applications it is desirable to provide external storage which the MCU can access while running and the user programmer can easily access at other times SD cards provide a cost effective solution which can be accessed by both the processor and user In practice these cards have file systems typically FAT and can be inserted in commonly available adaptors Furthermore the physical interface of most SD cards has a SPI mode described in the previous section and which is demonstrated with an actual implementation in Example 1 in the next section which 1s accessible Communicating with an SD memory card is relatively simple using some SPI driver like the one described in the previous section However controlling the card and interpreting the data communicated re
13. ion Use the provided uVision project lab8 sd dbg Clean and build then download to the MCB1700 board Observe operation and comment Study the source code The putty terminal should print useful information as in the figure below Note that the data read from the SD card matches the data shown using the HxD editor 10 MSD Init result 0 cardType 3D MsdBlockCount 1930240 MsdBlockSize 512 Byte MsdMemorySize 262 te MSD ReadBlo cki CMD17 acc MSD ReadBlock result data from SD Card result O da from SD Card result 1 from Sb Card result z from Sb Card result 3 from Sb Card result 4 from sD Care ult 5 from SD Card result 6 from SD Card result from Sb Card r ph 5 fram MEE t 9 from Sb Care uc 10 fram AREE ulti from sD Care ult le from Sb Card result 13 from Sb Card result 14 from Sb Card result 15 fram ae EAE oe from sD Care ult iF from Sb Card result i138 from Sb Card result 139 from Sb Card result Zz fram ee ult zi from sD Care ult z z2 from SD Card ult e3 Im apted co A re cn n m A i Im T F m IT m u rm to Im rU m WEN HH c mi H te Im T Im 1C nnn rn O oo m uy m m ul F Ei Ei Ei Ei Ei Ei Ei Ei Ei Ei Ei Ei Ei Ei Ei Ei Ei EI Ei Ei Ei Ei Ei Ei Im C A un ri m bor m T i n i n i n i Ti i T i T i T i n i n i T i T i ID iD D ID T
14. quires significant additional software Such software practically elevates the level of access to the card to a higher level of abstraction as illustrated in Fig 9 user or application space FAT Driver SD Driver SPI Driver SPI Module SPI BUS Figure 9 SD card software stack The aforementioned software is typically what is referred as a FAT file system driver One such example is the available widely used FatFs module which with some amount of porting could be utilized with our SPI driver Note that Keil offers a similar module too but it s not free if you tried to compile for example the SD File project you will get an error as you would need an RL license The data on an SD card is organized as a file system cards below 2GB are typically formatted as FAT16 file systems In a FAT file system the first several storage blocks are used to maintain data about the file system for example allocation tables while the remaining blocks are used to store the contents of files and directories An application which wishes to access data stored on an SD Card utilizes file level commands such as open read and write to access specific files within the SD card file system These commands are provided by a FAT file system driver The FAT file system issues commands at the level of block reads and writes without any knowledge of how these commands are implemented A separate SD driver implements these commands Finally the
15. remaining 32 bits are not important for this discussion a valid response indicates that your command was transmitted and processed successfully If successful the first 8 bits of the response will be either 000000010r 00000101 depending on the version of your SD card 39 38 37 36 35 34 33 32 31 28 27 1211 87 aa 0 0 Version Check Pattern in idle state erase reset illegal command CRC error erase sequence error address error parameter error Figure 7 The format of the 40 bit Response The steps necessary to complete the SD card initialization are shown in the flowchart in Fig 8 The flowchart consists of boxes in which a command number is specified Starting at the top of the flowchart each command has to be sent to the SD card and a response has to be received In some cases a response from the card needs to be processed to decide the next course of action as indicated by the diamond shaped decision boxes This is just one example of such an initialization sequence credit ftp ftp altera com up pub Altera_Material 12 0 Laboratory_Exercises Embedded_Systems DE2 embed_la b9 pdf from where portions of this discussion are adopted It s possible that 1t may work or not depending on the type of card you use and on its manufacturer Other similar such flowcharts exist for example http elm chan org docs mme sdinit png Most of the times you will have to try to implement several of them until you will
16. tt ID i D ID iD ID ID D ID D ID iD ID ID ID ID D ID iD IT E d d i 7 d d ul 7 d d i 7 u d EORR OH HB oH Ho Ho Ho Ho HoH Ho HH Ti C Hh 5 Example 2 Read picture from micro SD card and display on the 320x240 LCD display of the MCB1700 board This example is an improved version of the previous example Here I simply read the small bmp TODO make it work with png format too image alfaromeo bmp and display it on the 320x240 LCD screen of the MCB1700 board The program in this example can be found inside the uVision project lab5_sd_Icd inside the downloadable archive of lab 5 Clean and build then download to the MCB1700 board Observe operation and comment Study the source code You should see the image displayed on the LCD screen as shown below use the micro SD card from the TA 11 Figure 13 Cris Alfaromeo sports car shown on the LCD display of the MCB1700 board 6 Example 3 Manipulate files on the SD card using the FatFs file system The program in this example can be found inside the uVision project lab5 sd fat inside the downloadable archive of lab 5 Clean and build then download to the MCB1700 board Observe operation and comment Study the source code 7 References credits 1 SPI related references About SPI http elm chan org docs spi e html SPI bus see also the references and external links therein http en wikipedia org wiki Serial Peripheral Interface Bus 2 SD memory cards
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