AN1287 Using C18/HI-TECH C® Compiler to Interface Serial SRAM Devices to PIC16F/PIC18F Microcontrollers Author: This application note is part of a series that provide source code to help the user implement the protocol with minimal effort. Deepak Kumar Rana Microchip Technology Inc. INTRODUCTION Microchip’s serial SRAM product line represents a new way to add additional RAM to an application. With the small 8-pin packages and the SPI interface, these devices give designers added system flexibility. The 23XXXX series of serial SRAM devices from Microchip Technology support a half-duplex protocol that functions on a master-slave paradigm that is ideally suited to data stream applications. Figure 1 describes the hardware schematic for the interface between Microchip’s 23XXXX series devices and the PIC16F/18F series of MCUs. The schematic shows the connections necessary between either controller and the serial SRAM as tested, and the software was written assuming these connections. The HOLD pin is tied to VCC because this feature is not used in the examples provided. The bus is controlled by the Microcontroller (master), which accesses the 23XXXX using the MSSP peripheral built into the MCU configured for SPI operation. The MSSP peripheral can support throughput up to 5 or 8 MHz depending upon the device family of PIC16/18 MCU Selected. Communication can be paused using the HOLD pin. FIGURE 1: CIRCUIT FOR PIC16F887 AND 23XXXX SERIES DEVICE VCC CS VCC SO HOLD NC SCK VSS SI 23XXXX 15 16 17 18 23 24 25 26 19 20 21 22 27 28 29 30 8 9 10 U1 RE3/MCLR/VPP RC0/T1OSO/T1CKI RC1/T1OSI/CCP2 RC2/PA1/CCP1 RA0/AN0/ULPWU/C12IN0RC3/SCK/SCL RA1/AN1/C12IN1RA2/AN2/VREF-/CVREF/C2IN+ RC4/SDI/SDA RA3/AN3/VREF+/C1IN+ RC5/SDO RC6/TX/CK RA4/TOCKI/C1OUT RC7/RX/DT RA5/AN4/SS/C2OUT RA6/OSC2/CLKOUT RA7/OSC1/CLKIN RD0 RD1 RD2 RB0/AN12/INT RB1/AN10/C12IN3RD3 RB2/AN8 RD4 RB3/AN9/PGM/C12IN2RD5/P1B RB4/AN11 RD6/P1C RD7/P1D RB5/AN13/T1G RB6/ICSPCLK RE0/AN5 RB7/ICSPDAT RE1/AN6 RE2/AN7 1 2 3 4 5 6 7 14 13 33 34 35 36 37 38 39 40 PIC16F887 Note: CS and HOLD pins should all have pull-up resistors (~10k-ohms). See application note AN1245, “Recommended Usage of Microchip SPI Serial SRAM Devices.” © 2009 Microchip Technology Inc. DS01287A-page 1 AN1287 FIRMWARE DESCRIPTION The purpose of this application note is to offer the designer a set of examples for the read and write functions for using the Microchip SPI serial SRAM. Examples are included for the following modes: Byte, Page and Sequential Read and Writes. The code uses onchip MSSP hardware peripheral to communicate with the serial SRAM. The code was tested using the 23K256 SRAM mounted on the general purpose section of the PICDEM™ 2 Plus development board. The MSSP module is configured for SPI Master mode with a clock frequency of 5 MHz. The code is compatible with the PIC16F/18F families of MCUs with the MSSP module. Oscilloscope screen shots are shown in this application note. The following functions are provided to access the Serial SRAM. • • • • • • • • SRAMWriteStatusReg SRAMReadStatusReg SRAMWriteByte SRAMReadByte SRAMWritePage SRAMReadPage SRAMWriteSeq SRAMReadSeq The above functions are defined in the driver files SRAM_Driver.asm (for PIC16 assembly code and PIC18 C code) and SRAM_Driver.c (for PIC16 HITECH), with a respective SRAM_Driver.inc and SRAM_Driver.h files. The respective file pairs can be directly imported into the users application code. For the PIC18 C code we use the driver file in assembly (SRAM_Driver.asm) and call the assembly functions from C file Main_Demo.c. Four Code examples accompanying the application note are: • Assembly code example for PIC16 (MPASM™ assembler) • HI-TECH C® code example for PIC16 (HI-TECH C compiler) • C Code example for PIC18 (MPASM assembler and C18 compiler) • C Code example for PIC18 (HI-TECH C compiler) DS01287A-page 2 © 2009 Microchip Technology Inc. AN1287 INITIALIZATION WRITE STATUS REGISTER Only one function needs to be called to initialize the SRAM. To communicate with the SRAM we need to configure the on-chip MSSP module of the MCU for SPI operation. The InitSRAM() function initializes the MSSP module for SPI (Master mode 1, CKP=0, CKE=1) functionality. Using the on-chip MSSP peripheral makes the communication easier than bit-banging the I/O port, reducing software overhead. The default mode of operation for the serial SRAM is Byte mode and the user must select the appropriate mode (Byte, Page, Sequential) before the read or write operation. The functions that are provided configure the SRAM for the correct mode of operation, for example if a SRAMWritePage command is called, then the appropriate operating mode is selected. The STATUS register also has provision for enabling the HOLD feature, but this is not used in these examples. Figure 2 shows an example of the Write Status Register command. Chip Select is brought low (active) and the opcode is sent out through the SPI port. The Write Status command is given followed by the data to be written, in this case, Byte mode is selected. © 2009 Microchip Technology Inc. DS01287A-page 3 AN1287 FIGURE 2: WRITE STATUS REGISTER CS 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 0 SCK Instruction SI 0 0 0 0 Data to STATUS Register 0 0 0 1 7 6 5 4 3 2 High-Impedance SO DS01287A-page 4 © 2009 Microchip Technology Inc. AN1287 READ STATUS REGISTER Figure 3 shows an example of the Read Status Register command to check for the mode of operation and also the current status of the HOLD function. FIGURE 3: READ STATUS REGISTER CS 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 0 SCK Instruction SI 0 0 0 0 0 1 0 1 Data from STATUS Register High-Impedance SO © 2009 Microchip Technology Inc. 7 6 5 4 3 2 DS01287A-page 5 AN1287 BYTE WRITE The byte write operation consists of the following sequence: The Write command followed by the word address and data byte. The serial SRAM uses a 16-bit address, so two bytes must be transmitted for the entire word address, with the Most Significant Byte (MSB) first. FIGURE 4: Figure 4 shows an example of the Write command. For this, the device is selected and the opcode, 0x02, is sent. The High Address byte is given 0x00, followed by the Low Address byte, 0x20. Finally, the data is clocked in last, in this case, 0x55. BYTE WRITE COMMAND, ADDRESS AND DATA CS 0 1 2 3 4 5 6 7 8 9 10 11 21 22 23 24 25 26 27 28 29 30 31 SCK Instruction SI 0 0 0 0 0 16-bit Address 0 1 0 15 14 13 12 Data Byte 2 1 0 7 6 5 4 3 2 1 0 High-Impedance SO DS01287A-page 6 © 2009 Microchip Technology Inc. AN1287 BYTE READ The byte read operation can be used to read data from the serial SRAM. The MCU/DSC sends the command byte followed by the word address. Figure 5 shows an example of the Read command, followed by the MSB and LSB address bytes, followed by the read byte. FIGURE 5: BYTE READ (COMMAND BYTE, WORD ADDRESS AND READ BYTE) CS 0 1 2 3 4 5 6 7 8 9 10 11 21 22 23 24 25 26 27 28 29 30 31 SCK Instruction SI 0 0 0 0 0 16-bit Address 0 1 1 15 14 13 12 2 1 0 Data Out High-Impedance SO © 2009 Microchip Technology Inc. 7 6 5 4 3 2 1 0 DS01287A-page 7 AN1287 PAGE WRITE Page write operations provide a technique for increasing throughput when writing large blocks of data. The Serial SRAM features a 32-byte page. By using the page write feature, up to 1 full page of data can be written consecutively. It is important to point out that page write operations are limited to writing bytes within a single physical page regardless of the number of bytes being written. Physical page boundaries start at FIGURE 6: addresses that are integer multiples of the page size and end at addresses that are [integer multiples of the page size] – 1. Attempting to write across a page boundary results in the data being wrapped back to the beginning of the current page. Figure 6 shows Write command, address and data byte during a page write operation. PAGE WRITE (FIRST DATA BYTE) CS 0 1 2 3 4 5 6 Instruction SI 0 0 0 0 0 7 8 9 10 11 21 22 23 24 25 26 27 28 29 30 31 Page X, Word Y 16-bit Address 0 1 0 15 14 13 12 2 1 0 7 6 5 4 3 2 1 0 Page X, Word Y DS01287A-page 8 © 2009 Microchip Technology Inc. AN1287 PAGE READ Page read operations read a complete string, starting with the specified address. The page read operation also works similar to a page write operation and thus a maximum of 32 bytes can be read consecutively. Figure 7 shows an example of the entire sequence of commands necessary to perform the page read operation. For clarity, only the first byte is shown. FIGURE 7: PAGE READ (FIRST READ DATA BYTE) CS 0 1 2 3 4 5 6 7 8 9 10 11 21 22 23 24 25 26 27 28 29 30 31 SCK Instruction SI 0 0 0 0 0 16-bit Address 0 1 1 15 14 13 12 2 1 0 Page X, Word Y SO © 2009 Microchip Technology Inc. Page X, Word Y High Impedance 7 6 5 4 3 2 1 0 DS01287A-page 9 AN1287 SEQUENTIAL WRITE This operation is very useful while writing a long string which is more than the page size (32 bytes). This operation needs Write command (0x02) to be sent followed by upper address byte and lower address byte. The SRAM keeps writing data as long as it receives clock and valid data. When the last location of memory is reached, the next location that is written is the first address (0x0000), that is, the internal address counter rolls over. Figure 8 depicts the entire sequence of commands necessary to perform the sequential write operation. For clarity, only the first byte is shown. FIGURE 8: SEQUENTIAL WRITE (FIRST DATA BYTE) CS 0 1 2 3 4 5 6 7 8 9 10 11 21 22 23 24 25 26 27 28 29 30 31 SCK Instruction SI 0 0 DS01287A-page 10 0 0 0 16-bit Address 0 1 0 15 14 13 12 Data Byte 1 2 1 0 7 6 5 4 3 2 1 0 © 2009 Microchip Technology Inc. AN1287 SEQUENTIAL READ Sequential read operation allows the entire array to be read from the SRAM. The internal address counter automatically increments and page boundaries are ignored. When the internal address counter reaches the end of the array, the address counter will roll over to 0x0000. Figure 9 shows an example of the sequence of commands necessary to perform a sequential read operation. FIGURE 9: SEQUENTIAL READ (FIRST READ DATA BYTE) CS 0 1 2 0 0 0 3 4 5 6 Instruction SI 0 0 7 8 9 10 11 21 22 23 24 25 26 27 28 29 30 31 16-bit Address 0 1 1 15 14 13 12 2 1 0 Page X, Word Y SO © 2009 Microchip Technology Inc. 7 6 5 4 3 2 1 0 DS01287A-page 11 AN1287 CONCLUSION This application note offers designers a set of firmware routines to access SPI serial SRAM. The code demonstrates read and write operations for Byte, Page and Sequential modes. All the routines were written under the Microchip development environment MPLAB® IDE, using tools MPASM assembler, C18 and HI-TECH C compilers. The code was tested on Microchip’s PICDEM™ 2 Plus development board with the connections shown in Figure 1 with the PIC16F887 and PIC18F45K20. DS01287A-page 12 © 2009 Microchip Technology Inc. Note the following details of the code protection feature on Microchip devices: • Microchip products meet the specification contained in their particular Microchip Data Sheet. • Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions. • There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property. • Microchip is willing to work with the customer who is concerned about the integrity of their code. • Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as “unbreakable.” Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act. Information contained in this publication regarding device applications and the like is provided only for your convenience and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. MICROCHIP MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED, WRITTEN OR ORAL, STATUTORY OR OTHERWISE, RELATED TO THE INFORMATION, INCLUDING BUT NOT LIMITED TO ITS CONDITION, QUALITY, PERFORMANCE, MERCHANTABILITY OR FITNESS FOR PURPOSE. Microchip disclaims all liability arising from this information and its use. Use of Microchip devices in life support and/or safety applications is entirely at the buyer’s risk, and the buyer agrees to defend, indemnify and hold harmless Microchip from any and all damages, claims, suits, or expenses resulting from such use. No licenses are conveyed, implicitly or otherwise, under any Microchip intellectual property rights. Trademarks The Microchip name and logo, the Microchip logo, dsPIC, KEELOQ, KEELOQ logo, MPLAB, PIC, PICmicro, PICSTART, rfPIC and UNI/O are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. FilterLab, Hampshire, HI-TECH C, Linear Active Thermistor, MXDEV, MXLAB, SEEVAL and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A. Analog-for-the-Digital Age, Application Maestro, CodeGuard, dsPICDEM, dsPICDEM.net, dsPICworks, dsSPEAK, ECAN, ECONOMONITOR, FanSense, HI-TIDE, In-Circuit Serial Programming, ICSP, Mindi, MiWi, MPASM, MPLAB Certified logo, MPLIB, MPLINK, mTouch, Octopus, Omniscient Code Generation, PICC, PICC-18, PICDEM, PICDEM.net, PICkit, PICtail, PIC32 logo, REAL ICE, rfLAB, Select Mode, Total Endurance, TSHARC, UniWinDriver, WiperLock and ZENA are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. SQTP is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. © 2009, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. Microchip received ISO/TS-16949:2002 certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona; Gresham, Oregon and design centers in California and India. The Company’s quality system processes and procedures are for its PIC® MCUs and dsPIC® DSCs, KEELOQ® code hopping devices, Serial EEPROMs, microperipherals, nonvolatile memory and analog products. In addition, Microchip’s quality system for the design and manufacture of development systems is ISO 9001:2000 certified. © 2009 Microchip Technology Inc. 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