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TB026
ACCESSING MEMORY
The data EEPROM and FLASH Program memory are both accessed using the same method. An address and/or data value are stored in Special Function Registers (SFR) and then memory is accessed using control bits in other SFRs. There are six SFRs required to access memory: * * * * * * EECON1 EECON2 EEDATA EEDATH EEADR EEADRH
Calculating Program Memory Checksums Using a PIC16F87X
Rodger Richey Microchip Technology Inc.
INTRODUCTION www..com
Many applications require the microcontroller to calculate a checksum on the program memory to determine if the contents have been corrupted. Until now, the only family of PICmicro(R) microcontrollers to have the capability to read from program memory are the PIC17CXXX devices. The PIC16F87X devices are the first 14-bit core PICmicro microcontrollers that are able to access program memory in the same fashion as used with data EEPROM memory. These devices are FLASH extensions of the popular PIC16C7X family. Table 1 shows a comparison between the two PICmicro microcontroller families.
TABLE 1: PIC16C7X vs. PIC16F87X
Feature Pins Timers Interrupts Communication PIC16C7X 28 or 40 3 11 or 12 PSP, USART, PIC16C87X 28 or 40 3 13 or 14 PSP, USART, SSP( SPI or I2C Master/Slave) 20 MHz 10-bit 2 4K or 8K FLASH 192 or 368 bytes 128 or 256 bytes In-Circuit Debugger
When interfacing to data EEPROM memory, the address is stored in the EEADR register and the data is accessed using the EEDATA register. The operation is controlled using the EECON1 and EECON2 registers. The register map for EECON1 is shown in Figure 1. EECON2 is not a physical register. Reading it will result in all '0's. This register is used exclusively in the EEPROM and FLASH write sequences. When interfacing to FLASH program memory, the address is stored in the EEADRH:EEADR registers and the data is accessed using the EEDATH:EEDATA registers. Since the same set of control registers are used to access data and program memory, the EEPGD bit (EECON1<7>) is used to indicate to the microcontroller whether the operation is going to be on data memory (EEPGD = 0) or program memory (EEPGD = 1). Refer to Section 7.0 in the PIC16F87X data sheet (DS30292) for more information about using the EEPROM and FLASH memories.
SSP (SPI or I2C Slave) Frequency 20 MHz A/D 8-bit CCP 2 Program Mem. 4K or 8K EPROM RAM 192 or 368 bytes Data EEPROM None Other ---
(c) 1998 Microchip Technology Inc.
DS91026A-page 1
TB026
FIGURE 1:
R/W-x EEPGD bit7 U-0 --
EECON1 REGISTER
U-0 -- U-0 -- R/W-x WRERR R/W-0 WREN R/S-0 WR R/S-0 RD bit0
R W S U
= Readable bit = Writable bit = Settable bit = Unimplemented bit, read as `0' - n = Value at POR reset
bit 7:
EEPGD: Program / Data EEPROM Select bit 1 = Accesses Program memory 0 = Accesses data memory Note: This bit cannot be changed while a write operation is in progress. WRERR: EEPROM Error Flag bit 1 = A write operation is prematurely terminated (any MCLR reset or any WDT reset during normal operation) 0 = The write operation completed WREN: EEPROM Write Enable bit 1 = Allows write cycles 0 = Inhibits write to the EEPROM WR: Write Control bit 1 = initiates a write cycle. (The bit is cleared by hardware once write is complete. The WR bit can only be set (not cleared) in software. 0 = Write cycle to the EEPROM is complete RD: Read Control bit 1 = Initiates an EEPROM read (read takes one cycle. RD is cleared in hardware. The RD bit can only be set (not cleared) in software). 0 = Does not initiate an EEPROM read
bit 6:4: Unimplemented: Read as '0'
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bit 3:
bit 2:
bit 1:
bit 0:
DS91026A-page 2
(c) 1998 Microchip Technology Inc.
TB026
HEX FILE FORMAT
Development tools from Microchip support the Intel HEX Format (INHX8M), Intel Split HEX Format (INHX8S), and the Intel HEX 32 Format (INHX32). The most commonly used formats are the INHX8M and the INHX32. These are the only formats discussed in this document. Please refer to Appendix A in the MPASM User's Guide (DS33014) for more information about HEX file formats. The difference between INHX8M and INHX32 is that INHX32 supports 32-bit addresses using a linear address record. The basic format of the hex file is the same between INHX8M and INHX32 as shown below:
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The checksum provided by a programmer, such as PROMATE II or PICSTART PLUS, is not valid because the configuration word and device ID are included in the calculation. Therefore, a different program is required to calculate the program memory checksum. Once a complete HEX file has been obtained by the previously presented method, it must be processed and modified to contain the checksum. The program CHECKSUM.EXE, which is a DOS based program, reads in the HEX file, calculates the checksum, and outputs the new HEX file with checksum included. The checksum is calculated by: 1. 2. 3. 4. 5. Adding together the memory locations 0x0000 to 0x1FFE. Mask off all but the lower 14-bits. Take the 2's complement of Step 2. Mask off all but the lower 14-bits. Save this value into the HEX file at address 0x1FFF.
:BBAAAATTHHHH...HHHHCC
Each data record begins with a 9 character prefix and always ends with a 2 character checksum. All records begin with a ':' regardless of the format. The individual elements are described below. * BB - is a two digit hexadecimal byte count representing the number of data bytes that will appear on the line. * AAAA - is a four digit hexadecimal address representing the starting address of the data record. Format is high byte first followed by low byte, the address is doubled because this format only supports 8-bits (to find the real PICmicro address, simply divide the value AAAA by 2). * TT - is a two digit record type that will be '00' for data records, '01' for end of file records and '04' for extended address record (INHX32 only). * HHHH - is a four digit hexadecimal data word. Format is low byte followed by high byte. There will be BB/2 data words following TT. * CC - is a two digit hexadecimal checksum that is the two's complement of the sum of all the preceding bytes in the line record.
The program ignores all configuration word, ID, and EEPROM memory information in the HEX file and dumps it to the output file unchanged. The output file can then be programmed into the PIC16F87X device.
PICmicro Code
The code used by the PIC16F87X to calculate checksum uses 36 words of program memory and two data memory locations. The example code uses data memory locations 0x7E and 0x7F to store the calculated checksum. These locations are shared across all banks. The user can optionally change these locations and add banking into the routine. Figure 2 shows the flowchart for the routine. The checksum is created such that by adding up all program memory locations, a 14-bit result of 0x0000 is obtained. Since the calculation is done in 16-bits, the result will actually be 0x4000, but the upper two bits are masked off by the routine. Example 1 shows the code in MPASM to calculate the program memory checksum. If the program memory verifies, the routine returns a '1'. If a failure is detected, the routine returns a '0'.
HEX File Preparation
The checksum used to verify program memory contents is a 14-bit number calculated only on the program memory contents of a HEX file. The reason that only 14-bits is used is because the PIC16F87X has 14-bit wide program memory. The first step to obtaining the checksum is to get a complete HEX file that has all address locations specified. This can be easily accomplished in MPLAB by enabling the programmer, either PROMATE II or PICSTART PLUS, whichever one is available. Load the HEX file into MPLAB using the menus File -> Import -> Download to Memory. Then save the HEX file using File -> Export -> Save HEX File. Make sure that the Program Memory box is checked with a range of 0 to 8191 and the Configuration bits and IDs box are also checked. It is optional to check the EEPROM memory box depending on you application. This will create a complete HEX file including all program memory, configuration word, IDs, and optionally EEPROM memory.
(c) 1998 Microchip Technology Inc.
DS91026A-page 3
TB026
LISTING 1: PROGRAM MEMORY CHECKSUM ROUTINE
;Go to Bank 2 ;Clear the Checksum ;registers ;Set the Program Memory ; address to 0x0000 ;Loop for each location ; to read memory location ;Set for program memory ;Set for read operation ;Go to Bank 2 ;Add low byte to Checksum ;Check for overflow ;Yes, increment Checksum ;Add high byte ;Increment low address ;Check for overflow ;Increment high address ;Check to see if ; address wrapped ; from 0x1fff to ; 0x0000 CalcChecksum bsf STATUS,RP1 bcf STATUS,RP0 clrf ChecksumL clrf ChecksumH clrf EEADR clrf EEADRH CLoop bsf STATUS,RP0 bsf EECON1,EEPGD bsf EECON1,RD bcf STATUS,RP0 nop movf EEDATA,W addwf ChecksumL,F btfsc STATUS,C www..com incf ChecksumH,F movf EEDATH,W addwf ChecksumH,F incf EEADR,F btfsc STATUS,Z incf EEADRH,F movf EEADRH,F btfss STATUS,Z goto CLoop movf EEADR,F btfss STATUS,Z goto CLoop bcf bcf movf btfss retlw0 movf btfss retlw0 retlw1 ChecksumH,7 ChecksumH,6 ChecksumH,F STATUS,Z ChecksumL,F STATUS,Z ;Checksum failed ;Checksum passed
;Checkcum calculation complete ;Clear upper 2 bits ; only 14-bit checksum ;Checksum should be 0 ;Checksum failed
DS91026A-page 4
(c) 1998 Microchip Technology Inc.
TB026
FIGURE 1: FLOWCHART
Calculate Checksum
Initialize Checksum registers to 0x00
Initialize EEADRH:EEADR registers to 0x0000
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Read a word out of program memory
Add EEDATA to Checksum
Z = 1? Yes Increment Checksum high byte
No
Add EEDATH to Checksum
Increment EEADR
Z = 1? Yes Increment EEADRH
No
No
EEADRH: EEADR = 0? Yes Mask off upper two bits of Checksum
Checksum = 0? Yes Return 1 in W
No
Return 0 in W
(c) 1998 Microchip Technology Inc.
DS91026A-page 5
WORLDWIDE SALES AND SERVICE
AMERICAS
Corporate Office
Microchip Technology Inc. 2355 West Chandler Blvd. Chandler, AZ 85224-6199 Tel: 480-786-7200 Fax: 480-786-7277 Technical Support: 480-786-7627 Web Address: http://www.microchip.com Microchip Technology Inc. 500 Sugar Mill Road, Suite 200B Atlanta, GA 30350 Tel: 770-640-0034 Fax: 770-640-0307
AMERICAS (continued)
Toronto
Microchip Technology Inc. 5925 Airport Road, Suite 200 Mississauga, Ontario L4V 1W1, Canada Tel: 905-405-6279 Fax: 905-405-6253
ASIA/PACIFIC (continued)
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ASIA/PACIFIC
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EUROPE
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11/15/99
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Microchip received QS-9000 quality system certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona in July 1999. The Company's quality system processes and procedures are QS-9000 compliant for its PICmicro(R) 8-bit MCUs, KEELOQ(R) code hopping devices, Serial EEPROMs and microperipheral products. In addition, Microchip's quality system for the design and manufacture of development systems is ISO 9001 certified.
All rights reserved. (c) 1999 Microchip Technology Incorporated. Printed in the USA. 11/99
Printed on recycled paper.
Information contained in this publication regarding device applications and the like is intended for suggestion only and may be superseded by updates. No representation or warranty is given and no liability is assumed by Microchip Technology Incorporated with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Use of Microchip's products as critical components in life support systems is not authorized except with express written approval by Microchip. No licenses are conveyed, implicitly or otherwise, under any intellectual property rights. The Microchip logo and name are registered trademarks of Microchip Technology Inc. in the U.S.A. and other countries. All rights reserved. All other trademarks mentioned herein are the property of their respective companies.
1999 Microchip Technology Inc.

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