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Microchip Technology - Calculating Program Memory Checksums Using a PIC16F87X

Numéro de référence TB026
Description Calculating Program Memory Checksums Using a PIC16F87X
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TB026
Calculating Program Memory Checksums Using a PIC16F87X
Author: Rodger Richey
Microchip Technology Inc.
www.DataSheet4U.comINTRODUCTION
Many applications require the microcontroller to cal-
culate a checksum on the program memory to deter-
mine if the contents have been corrupted. Until now,
the only family of PICmicro® 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
PIC16C7X
Pins
28 or 40
Timers
3
Interrupts
11 or 12
Communica-
tion
PSP, USART,
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
---
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
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 Reg-
isters (SFR) and then memory is accessed using con-
trol bits in other SFRs. There are six SFRs required to
access memory:
• EECON1
• EECON2
• EEDATA
• EEDATH
• EEADR
• EEADRH
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 regis-
ters. 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.
© 1998 Microchip Technology Inc.
DS91026A-page 1

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