Configuring watchdog timeout in Embedded C - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

Watchdog Timer Timeout Value Calculation

Given a watchdog timer with a clock frequency of 1 MHz and a prescaler of 64, what is the timeout period in milliseconds if the watchdog counter is set to 250?

Embedded C

/* Timeout (ms) = (Counter * Prescaler) / (Clock Frequency in kHz) */
// Clock Frequency = 1 MHz = 1000 kHz
// Prescaler = 64
// Counter = 250

unsigned int timeout_ms = (250 * 64) / 1000;

A 64 ms

B 250 ms

C 16 ms

D 4 ms

💡 Hint

Calculate the timeout by multiplying the counter by the prescaler, then divide by the clock frequency in kHz.

Timeout in milliseconds = (Counter * Prescaler) / (Clock frequency in kHz). Here, (250 * 64) / 1000 = 16 ms.

❓ Predict Output

intermediate

Watchdog Configuration Register Value

If a watchdog timer register uses bits 0-3 to set the prescaler value as a power of two (e.g., 0 means divide by 1, 1 means divide by 2, 2 means divide by 4, etc.), what value should be written to set a prescaler of 128?

Embedded C

/* Prescaler = 2^n, find n for prescaler 128 */
// 2^7 = 128

unsigned char prescaler_setting = 7;

A 6

B 7

C 8

D 128

💡 Hint

Find the power n such that 2^n equals the prescaler value.

Since 2^7 = 128, the register bits should be set to 7 to get a prescaler of 128.

🔧 Debug

advanced

Identify the Bug in Watchdog Timeout Setup

What error does the following code produce when configuring the watchdog timeout? unsigned int timeout = 5000; void setup_watchdog() { WDT_CTRL = timeout << 2; } Assume WDT_CTRL is an 8-bit register and timeout is in milliseconds.

Embedded C

unsigned int timeout = 5000;
void setup_watchdog() {
    WDT_CTRL = timeout << 2;
}

A No error, code runs correctly

B SyntaxError due to missing semicolon

C TypeError because timeout is unsigned int

D Value overflow causing incorrect timeout

💡 Hint

Consider the size of WDT_CTRL and the value assigned to it.

Shifting 5000 (which is 0x1388) left by 2 bits results in a value too large for an 8-bit register, causing overflow and incorrect timeout setting.

📝 Syntax

advanced

Correct Syntax for Watchdog Timeout Macro

Which option correctly defines a macro to calculate watchdog timeout in milliseconds given clock frequency (in Hz), prescaler, and counter value?

Embedded C

#define WDT_TIMEOUT_MS(clock, prescaler, counter) ???

A #define WDT_TIMEOUT_MS(clock, prescaler, counter) ((counter) * (prescaler) * 1000 / (clock))

B #define WDT_TIMEOUT_MS(clock, prescaler, counter) (counter * prescaler / clock * 1000)

C #define WDT_TIMEOUT_MS(clock, prescaler, counter) ((counter * prescaler) / (clock / 1000))

D #define WDT_TIMEOUT_MS(clock, prescaler, counter) ((counter + prescaler + 1000) / clock)

💡 Hint

Use parentheses to ensure correct order of operations and multiply by 1000 to convert seconds to milliseconds.

Option A correctly multiplies counter and prescaler, then multiplies by 1000, and divides by clock frequency, all with proper parentheses.

🚀 Application

expert

Calculate Maximum Watchdog Timeout

An embedded system uses an 8-bit watchdog timer counter and a prescaler selectable as powers of two from 1 to 256. The clock frequency is 2 MHz. What is the maximum watchdog timeout in milliseconds achievable?

Embedded C

/* Max counter = 255 (8-bit)
   Max prescaler = 256
   Clock frequency = 2 MHz = 2,000,000 Hz
   Timeout (s) = (counter * prescaler) / clock
   Timeout (ms) = Timeout (s) * 1000
*/

A 32.64 ms

B 326.4 ms

C 652.8 ms

D 65.28 ms

💡 Hint

Calculate timeout in seconds first, then convert to milliseconds.

Timeout = (255 * 256) / 2,000,000 = 0.03264 seconds = 32.64 ms. But careful: 255 * 256 = 65280, so 65280 / 2,000,000 = 0.03264 s = 32.64 ms. The correct option is 32.64 ms (A).