Fixing Power-on Reset Circuit Failures in PIC16F18854-I/ML
Introduction: The PIC16F18854-I/ML is a popular microcontroller from Microchip, and like all microcontrollers, it requires a stable power-on reset (POR) circuit to initialize its internal registers and peripherals correctly. A failure in the power-on reset circuit can cause the microcontroller to malfunction or not start at all. In this analysis, we’ll explore the potential causes of POR circuit failures and how to solve them in a clear, step-by-step manner.
1. Possible Causes of Power-on Reset Failures
There are several factors that could lead to a failure of the power-on reset circuit in the PIC16F18854-I/ML. Below are the most common causes:
1.1 Incorrect Reset Pin Configuration (MCLR) The MCLR (Master Clear) pin is used to initiate a reset. If this pin is incorrectly configured (for example, configured as a digital input instead of a reset input), the microcontroller may not properly initiate the power-on reset. Cause: Incorrect configuration or floating MCLR pin. 1.2 Insufficient Power Supply Voltage The PIC16F18854 requires a certain minimum voltage to perform a proper reset. If the supply voltage is too low or unstable, the reset circuit may not function correctly. Cause: Power supply failure or improper voltage level. 1.3 Faulty Reset Circuit Components The reset circuit often includes components like resistors, capacitor s, or external reset ICs (such as the Supervisory Reset IC). Any of these components could be faulty or improperly connected. Cause: Faulty components or bad soldering connections. 1.4 Incorrect Timing of Reset Signal The reset signal needs to be asserted for a sufficient duration at power-up. If this duration is too short, the microcontroller may fail to reset properly. Cause: Poor capacitor size or incorrect resistor value in the reset timing circuit. 1.5 Brown-Out Reset Configuration If the microcontroller is configured to activate the brown-out reset (BOR) feature, any voltage drop below a specific threshold could trigger a reset. If the BOR threshold is set incorrectly or the power supply has spikes or dips, this can interfere with the reset process. Cause: Incorrect BOR configuration or unstable power.2. Troubleshooting the Power-on Reset Circuit
Once you identify the possible causes, it’s important to follow a systematic approach to fix the issue. Here’s a step-by-step guide to troubleshooting and resolving the failure:
2.1 Step 1: Check the MCLR Pin Configuration Action: Confirm that the MCLR pin is configured as a reset pin (not as a digital input). In the PIC16F18854, the MCLR pin should be enabled to trigger a reset on power-up. How to Fix: If it’s incorrectly configured, adjust the configuration settings in the firmware or re-wire the pin accordingly. 2.2 Step 2: Verify the Power Supply Voltage Action: Measure the supply voltage using a multimeter to ensure it is within the recommended operating range (typically 1.8V to 3.6V for the PIC16F18854). How to Fix: If the voltage is low or unstable, check your power supply circuit for issues like inadequate filtering, damaged components, or incorrect voltage regulators. You may need to replace or adjust the power supply to ensure stable voltage. 2.3 Step 3: Inspect Reset Circuit Components Action: Inspect the external reset circuitry, including resistors, capacitors, and external reset ICs (if used). These components should be connected as per the reference circuit in the PIC16F18854 datasheet. How to Fix: If any component is damaged or incorrectly placed, replace it with a new component. Double-check connections and solder joints for cold soldering or loose connections. 2.4 Step 4: Check the Timing of the Reset Signal Action: Ensure that the reset signal remains active for a sufficient duration (usually around 50ms to 200ms) after power is applied. This is controlled by the resistor and capacitor values in the reset circuit. How to Fix: If the reset signal duration is too short, adjust the values of the resistor and capacitor. For example, increasing the capacitor size will lengthen the reset pulse duration. Refer to the datasheet for appropriate values. 2.5 Step 5: Verify the Brown-Out Reset (BOR) Configuration Action: Check if the BOR feature is enabled and set to an appropriate voltage threshold. In the PIC16F18854, the BOR can be configured to reset the device if the voltage dips below a certain level. How to Fix: If the BOR is incorrectly configured, modify the settings in the microcontroller’s configuration registers to match the desired threshold, ensuring it is appropriate for the operating voltage range.3. Preventive Measures
Once the issue has been resolved, consider taking the following preventive measures to avoid similar problems in the future:
Use a Power-On Reset IC: You may consider using an external Power-On Reset IC, which can provide a reliable reset signal independent of the microcontroller's internal reset circuit. Ensure Stable Power Supply: Always use a well-regulated and filtered power supply to ensure clean and stable voltage levels. Design Robust Reset Circuitry: Follow recommended guidelines for reset circuitry, ensuring that the reset signal is both reliable and timely.Conclusion
Power-on reset circuit failures in the PIC16F18854-I/ML can be caused by various factors, including incorrect MCLR pin configuration, insufficient power supply voltage, faulty reset circuit components, incorrect reset timing, or issues with the brown-out reset feature. By following a systematic approach to troubleshoot and fix these issues—checking the MCLR pin, verifying the power supply, inspecting the reset components, and adjusting the reset signal timing—you can resolve these failures effectively.
