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Text Script for eLearning course "Resets"
Table of contents
Topic Slide Number
Introduction 1
Reset Mechanisms 2
Reset Status bits 3
Power-on Reset 4
Brown-out Reset 5
Knowledge Check #1 6
MCLR Pin 7
Driving MCLR 8
MCLR Effects 9
Watch-dog Timer 10
Knowledge Check #2 11
Closing 12
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Slide 1: Introduction
Microchip Technology welcomes you to the Reset module of the PICmicro x14
microcontroller. This module will give you a basic overview of the PICmicro MCU
Resets and is intended for those new to PICmicro MCU or requiring a refresher
on Resets. It is recommended you have previously taken or are familiar with the
PIC MCU x14 architecture and instruction set modules.
Slide 2: Reset Mechanisms
There are four kinds of reset mechanisms fo r the PIC MCU x14 architecture. The
first two have to do with resetting the part based on power to the device. They
are Power-On Reset, which occurs on start-up, and Brown-Out Reset, which
holds the part in reset if the Vdd value drops below a safe level.
You can also force a reset by using the MCLR pin and lastly, the internal
Watchdog Timer can cause a reset if it is allowed to time out.
This presentation will discuss each of these reset mechanisms in detail.
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Slide 3: Reset Status Bits
Before we discuss the details of each reset mechanism, we should point out
some status bits that are important for this discussion. Availability of these bits is
product dependent, so please look at the datasheet for the product you are
planning on using.
There are 2 bits in the status register and 2 bits in the Power Control and Status
(PCON) register that can be used to determine what event caused a reset
condition.
In the Status register, we are interested in the Time-out (TO) bit and the Power-
down (PD) bit.
The Time-out (TO) bit is set to a 1 after power-up, a CLRWDT instruction or a
Sleep instruction. The TO bit is only cleared to a 0 when a WDT timeout has
occurred.
The Power-down (PD) bit is set high after a power-up or the execution of the
CLRWDT instruction. This bit will be cleared low only by the execution of the
sleep instruction.
In the Power Control (PCON) register, we use the Power-on Reset Status (POR)
bit and the Brown-out Reset (BOR) Status bit. These 2 flag bits in conjunction
with the TO and PD bits in the Status Register can be used to differentiate
between a Power-on Reset (POR), Brown-out Reset (BOR), a Watchdog Timer
Reset (WDT) and an external MCLR Reset.
If the Power-on Reset Status (POR) bit is low, it indicates that a Power-on Reset
has occurred. This bit must be set to 1 in software after the event has occurred.
Likewise, if the Brown-out Reset Status bit is low, it means that a Brown-out
Reset has occurred. This bit must also be reset in software.
This table that shows the reset status bits can be found in most device
datasheets as well as the PICmicro Mid-Range MCU Reference Manual
(Show the table `Status Bits and their significance')
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Slide 4: Power On Reset (POR)
The first reset mechanism we will discuss is the Power-On Reset circuit, which is
present on all PICmicro