Watchdog Timer

The Basics

Table of Contents

Reference(s):

Material in this document was drawn from these three sources

  1. Watchdog Timer Basic Example, Written by Nicolas Larsen, 10 June 2011
  2. ATmega48PA/88PA/168PA/328P Section 10.8 Watchdog Timer (page 50 / 448)
  3. ATmega16U4/ATmega32U4 Section 8.2 Watchdog Timer (page 48 / 448)
  4. Standard C library for AVR-GCC avr-libc wdt.h library
  5. You can find the wdt.h file in the Arduino\hardware\tools\avr\avr\include\avr folder

Introduction

 The watchdog timer watches over the operation of the system. This may include preventing runaway code or in our C example, a lost communications link.

  The watchdog timer operates independent of the CPU, peripheral subsystems, and even the clock of the MCU.

 To keep the watchdog happy you must feed it a wdr (watchdog reset) assembly instruction before a predefined timeout period expires.

The timeout period is defined by a ~128KHz watchdog timer clock and a programmable timer.

Watchdog Timer Reset

“In normal operation mode, it is required that the system uses the WDR – Watchdog Timer Reset – instruction to restart the counter before the time-out value is reached. If the system doesn’t restart the counter, an interrupt or system reset will be issued.” ATmega328P Datasheet Section 10.8.2 Overview

When the Watchdog Reset (wdr) instruction is encountered (pun intended), it generates a short reset pulse of one CK cycle duration. On the falling edge of this pulse, the delay timer starts counting the Time-out period tTOUT.

Figure 1. Watchdog Timeout

Watchdog Timer Module

To configure the watchdog timer you define the timeout period by setting the pre-scale value, and define action to be taken if a timeout occurs.

Figure 2. Watchdog Prescaler

Configuration bits are found in the WDTCSR – Watchdog Timer Control Register. Before you can change the WDE and/or prescaler bits (WDP3:0), the WDCE – Watchdog Change Enable bit must be set.

Figure 3. WDTCSR

Define Timeout Period

The WDP3..0 bits determine the Watchdog Timer prescaling when the Watchdog Timer is running. The different prescaling values and their corresponding time-out periods are shown here.

Figure 4. WDP Bit Definition

On your own…
How many flip-flops are needed to implement the watchdog prescaler?
Hint: How many bits are needed to generate the longest delay with an input clock frequency of 128KHz?

Define Timeout Action

The Watchdog always on (WDTON) fuse, if programmed, will force the Watchdog Timer to System Reset mode. With the fuse programmed (WDTON = 0) the System Reset mode bit (WDE) and mode bit (WDIE) are locked to 1 and 0 respectively. Arduino / ATmega 328P fuse settings.

The Arduino ATmega328P bootloader sets the fuse to unprogrammed WDTON = 1, which means you can program the action to be taken by setting or clearing the WDE and WDIE bits as shown in the following table.

Figure 5. WDT Modes

Note: 1. WDTON Fuse set to “0” means programmed and “1” means unprogrammed.

Watchdog Timer is in Interrupt and System Reset Mode – When the interrupt occurs the hardware automatically clears the WDIE bit, thus putting the watchdog timer into the “System Reset” mode, as defined in the table (WDTON = 1, WDIE = 0, WDE = 1). At the next timeout, a reset is generated.

Watchdog Setup

The following assembly and C code examples start the watchdog timer in System Reset Mode with a timeout period of ~0.5 seconds.

Assembly Code Example

C Code Example

Macros may be found in the avr/wdt.h library.

Turning Off the Watchdog Timer

Assembly Code Example

C Code Example

3DoT Watchdog

Setup

In the watchdogSetup C++ program the WDTCSR register is configured to operate the watchdog timer in the “Interrupt and System Reset” or “Interrupt” mode, with a programmable delay from 1 to 8 seconds.

To configure the WDT a 0x10 WATCHDOG_SETUP command packet is sent with one of the following arguments.

Argument
Mode
Timeout
0x00
Watchdog Off
0x4E
Interrupt and System Reset
1 sec
0x4F
2 sec
0x68
4 sec
0x69
8 sec
0x46
Interrupt Only
1 sec
0x47
2 sec
0x60
4 sec
0x61
8 sec

0x0E Exception CodesIf one of these arguments is not sent the program sends a 0x0E “Exception” packet with a 0x06 “Watchdog timeout out of range” code. To put this in perspective, here are all the Exception codes and what they mean.

High Low Order Byte
01 Start byte 0xA5 expected
02 Packet length out of range 1 – 20
03 LRC checksum error
04 Undefined command decoder FSM state
05 Array out of range i >= 23
06 Watchdog timeout out of range

Timeout

If programmed for 8 second “Interrupt and Reset” Mode and a “WDR” command packet is not sent within the timeout period, an interrupt will occur at T+8 seconds and system reset at T+16.

When the interrupt occurs T+8 seconds the hardware automatically clears the WDIE bit, thus putting the watchdog timer into the “System Reset” mode (WDTON = 1, WDIE = 0, WDE = 1).

  • A 0x0B “Emergency” telemetry packet with 0x0100 code is sent.
0x0B Emergency Code
High Low 
01   00  Watchdog timeout
  • After this interrupt, at any time (up to T+16) you can reset the timer, turn it off, change modes, etc.

Demonstration

  • Plug in an Arduino UNO
  • Launch and Configure CoolTerm
  • Launch arxrobot_firmware_3DoT
  • Normal Operation
A5 02 10 69 DE    Set watchdog interrupt for 8 sec
A5 01 11 B5       Ping (repeat at a frequency of less than 0.125 seconds)
CA 01 11 DA       Pong
A5 02 10 00 B7    Turn Watchdog Off

• Timeout Example

A5 02 10 4E 59     Set watchdog interrupt for 1 sec
CA 03 0B 01 00 C3  Emergency Code 0B, Watchdog timeout 0100
CA 03 06 00 63 AC  Read and transmit sensor values after restart
CA 03 02 00 00 CB

• Timeout Prescaler out-of-range

A5 02 10 62 D5     ATmega reserved
CA 03 0E 06 62 A3
      ↓   |  ↓ 
exception |  argument
error     ↓ 
watchdog timeout out of range

3DoT C++ Watchdog Object

The 3DoT Watchdog object has only one public method

void watchdogSetup(uint8_t);

The 3DoT Watchdog object has two private methods

void watchdogOff();
void throwError(uint16_t);

The 3DoT Watchdog object has three read-only private properties

uint8_t _prescaler;
uint8_t _mode;
uint8_t _counter;

In the next section we take a closer look at the watchdogSetup method.

3DoT Watchdog C++ Code

Review Questions

  1.  TBD

Answers

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