Configuration Bits are used to enable or disable different hardware features of the microcontroller. These are read during startup/reset and enable or disable different hardware features based on the values of bits. Configuration bits in pic16f877a can be used to control the hardware features of pic such as clock source, Power-up Timer, Watchdog Timer, Memory Code Protection, etc. The configuration bits are mapped to the program memory location 2007h. it belongs to the configuration memory space, not the user program memory space and hence it can be programmed only during the programming time.

The configuration bits are set using the directives written in the software. The configuration bit syntax can be understood from the compiler manual. The user can easily create the configuration code using the IDE such as MPLAB X IDE. The configuration bit window in the IDE can be used to generate suitable directives according to the selected options. The tutorial discusses different configuration bits in PIC16F877A and the method to generate desired directives using MPLAB X IDE.

Configuration bits in PIC16F877A

Oscillator Configurations

The Oscillator provides the required clock signal for the operation of the microcontroller. The pic16f877a has four oscillator modes namely LP, XT, HS, and RC. The modes LP, XT, HS utilizes a crystal or a ceramic resonator and it is connected to the clock input pins to generate the required osculations. The LP mode is designed to drive 32KHZ Crystal. The XT mode is selected for the crystals in the range of 4MHZ or lower and in the case of HS mode the crystal range is above 4MHZ. The RC Mode is selected when the RC network is used to create the clock input for the MCU. The oscillation frequency accuracy is less in RC Network. It can be used in time-insensitive applications for cost-saving.

Watchdog Timer (WDT)

Watchdog timer detects when the microcontroller is stuck in an endless loop. It is a free-running counter that generates devices reset when the WDT Time out occurs. In the sleep mode operation, WDT time out wake up the device to continue its operation. The program writes zero every time. Watchdog timer detects the software malfunctioning and resets the controller. watchdog timer counts from an initial value set by the controller to the zero value. The embedded software restarts the WDT Before reaching zero. In the software malfunctioning events, the counter reaches zero and it generates the reset signal.

Power-up Timer (PWRT)

The power-up timer introduces a small delay after power-up reset or brown-out reset. The quick start of the microcontroller after the rest can cause problems. The power-up timer provides a startup delay of 72 ms which allows VDD to rise to an acceptable level. The reset state of the chip is retained as long as the PWRT is active. The PWRT ensures the stability of the supply voltage before the clock gets started.

Brown-out Reset (BOR)

Brown out reset allows the microcontroller to reset when the supply voltage drops below a specific voltage. IF the VDD falls below VBOR(~4V) For a duration of TBOR (100 µS) it reset the device. The device will remain in a reset state in the BROWN out reset case until the VDD again rises above VBOR and after that, the Power-up Timer keeps the device in reset for a time duration of TPWRT ( 72 mS). The device which falls below VBOR In the TPWRT initiates BROWN out restart when VDD rises above VBOR with the Power-up Timer Reset. Regardless of the state of the PWRT configuration bit, the power-up timer is enabled with the Brown-out Reset activation.

Low Voltage (SINGLE SUPPLY) ICSP

The LVP bit is for enabling low voltage programming mode. The advantage of low voltage programming is that the device can be programmed with ordinary logic levels whereas, in high voltage programming, the MCLR pin should be connected to the voltage level more than Vdd. In low voltage programming mode, it uses only a single supply. To enter programming mode, PGM bit should be connected to Vdd, so the bit PGM (RB5) will not be available for I/O operation.

Data EEPROM Memory Code Protection bit (CPD)

The two bits CPD and WRTD protects the EEPROM Data from external reads and writes.CPD controls internal and external reads and writes whereas WRTD controls internal reads and writes.

Flash Program Memory Write Enable bit

These bits allow as to select the sectors in the flash programming memory for recording of the data or for the In-circuit Serial Programming. WE can write to the flash memory by the use of EECON. The firmware can be used to write data into the flash memory.

Flash Program Memory Code Protection bit

This bit protests the code that is stored in the flash programming memory. The device firmware can be protected from external read using this method.

Configuration Word Register

The 14-bit configuration word register is located in the program memory space 2007h. it resides in the flash memory and it is beyond the user program memory space and hence only be programmed during the device programming time. The unprogrammed value of this register is 3FFFh. Refer to the pic16f877a datasheet for more details.

pic configuration bits and configuration word for pic16f877a

 

Procedure for setting Configuration bits in PIC16F877A

MPLAB users can edit these configuration bits by using the Configuration Bits tool.



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