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Timer Prescaler Calculator

EMBEDDED & LOW-LEVEL · FREE DEVELOPER TOOL October 2026 11 min read

Finds a prescaler and auto-reload pair for a target timer frequency, with resolution, error and register-range checks. Free, instant and entirely client-side.

For a given target, a smaller prescaler gives finer resolution but requires a larger ARR; choose the largest prescaler that still fits the ARR range.

Finds a prescaler and auto-reload pair for a target timer frequency, with resolution, error and register-range checks.

EMBEDDED & LOW-LEVELClient-sideWorked examples2026
Common Cortex-M0 clock
16 MHz
Source: STM32 / nRF52 class
Standard UART baud rate
115200
Source: 8N1 serial
Fast-mode I2C clock
400 kHz
Source: NXP UM10204
Classical CAN bit rate
1 Mbit/s
Source: ISO 11898-1

Quick answer

Finds a prescaler and auto-reload pair for a target timer frequency, with resolution, error and register-range checks. The calculator applies f_timer = clock / (prescaler x (arr + 1)); period = 1 / f_timer and shows every intermediate value.

On this page
  1. 1Quick Answer
  2. 2Interactive Calculator
  3. 3Timer Prescaler Calculator: Calculation Breakdown
  4. 4Key Takeaways
  5. 5Embedded Programming & Low-Level Computing Reference Numbers
  6. 6Implementation Approaches
  7. 7Reference Tables
  8. 8Formulas & How to Use
  9. 9Worked Examples
  10. 10Advanced Guide: Timer Prescaler Calculator
  11. 11Language Playbooks
  12. 12Glossary
  13. 13Mistakes & Pro Tips
  14. 14Use Cases
  15. 15Related Calculators
  16. 16Long-Tail Answers
  17. 17FAQs (28)
  18. 18References

Interactive Calculator

Enter a value — every result updates instantly, in your browser. Free forever, no sign-up.

Private Exact math

Timer Prescaler Calculator

Every input is evaluated locally in your browser with deterministic arithmetic.

Runs in your browser
Timer clock (Hz) Input clock to the timer counter.Target frequency (Hz) Desired update or interrupt rate.Counter width Auto-reload value (0 = auto) Set a known ARR to check it, or leave 0 to choose one automatically.Maximum prescaler Hardware limit for the prescaler register.

Live result

PSC 2, ARR 41,999

PSC 2, ARR 41,999, 1,000 Hz (0% error).

PSC 2ARR 41,9991,000 Hz

Timer clock

84 MHz

Target frequency

1,000 Hz

Counter width

16 bits

Prescaler (PSC)

2

Timer Prescaler Calculator: Calculation Breakdown

Every input, intermediate value and result row behind the estimate.

Calculated values and supporting details
Timer clock
84 MHz
Target frequency
1,000 Hz
Counter width
16 bits
Prescaler (PSC)
2
Auto-reload (ARR)
41,999
Actual frequency
1,000 Hz
Frequency error
0%
Period
1 millisecond
Counter resolution
23.80952381 nanoseconds
Total divider
84,000
Maximum period at this prescaler
1 millisecond
Maximum possible period
51.1298 seconds
Register check
Fits hardware limits

Valid prescaler and ARR pairs

PrescalerARRActual HzError
241,9991,0000%
420,9991,0000%
810,4991,0000%
165,2491,0000%
322,6241,0000%
641,312999.619-0.038%
1286551,000.3810.038%
2563271,000.3810.038%
1024811,000.3810.038%

Step-by-step calculation

  1. 1Ideal total divider = 84,000,000 / 1,000 = 84,000.
  2. 2Choose PSC 2 and ARR 41,999: divider = 2 x (41,999 + 1) = 84,000.
  3. 3Actual frequency = 84,000,000 / 84,000 = 1,000 Hz, an error of 0%.
  4. 4Each counter tick is 23.80952381 nanoseconds, and the full ARR range spans 1 millisecond.

Key Takeaways

The 30-second version — the exact answers Google loves to feature.

  • 1For a given target, a smaller prescaler gives finer resolution but requires a larger ARR; choose the largest prescaler that still fits the ARR range.
  • 2The timer update event occurs after ARR + 1 counts, so the divider is prescaler x (ARR + 1).
  • 3A 16-bit timer at 84 MHz cannot reach 1 Hz directly; it needs a prescaler of at least 1282.
  • 4Formula: f_timer = clock / (prescaler x (ARR + 1)); period = 1 / f_timer
  • 5Default scenario: PSC 2, ARR 41,999

Embedded Programming & Low-Level Computing Reference Numbers

Useful benchmarks to sanity-check the output before you commit to a decision.

Common Cortex-M0 clock
16 MHz
Source: STM32 / nRF52 class
Standard UART baud rate
115200
Source: 8N1 serial
Fast-mode I2C clock
400 kHz
Source: NXP UM10204
Classical CAN bit rate
1 Mbit/s
Source: ISO 11898-1
Typical RTOS tick
1 ms
Source: FreeRTOS / Zephyr
Local calculations
100%
Source: No hardware access

Implementation Approaches

Three practical ways teams use timer prescaler calculator, with the precision and safeguards each one needs.

Recommended

Datasheet-first timing

Best for: UART, SPI, I2C, CAN and timer configuration

  • Method: Start from the peripheral clock, prescaler and protocol overhead in the datasheet, then compute the nominal bit time and the worst-case error.
  • Precision: Use the minimum and maximum oscillator tolerance, not the typical value, and include divider rounding.
  • Safeguards: Verify the result with a logic analyzer or oscilloscope on real hardware before shipping.

Worst-case memory budget

Best for: Firmware images, stacks, heaps and RAM reservation

  • Method: Add code, read-only data, initialized data and zero-initialized data from the map file, then reserve stack from the deepest call chain and ISR nesting.
  • Precision: Measure stack high-water marks at peak load and keep the largest observed value as the baseline.
  • Safeguards: Leave at least 20-30% headroom in flash and RAM, and fail the build when the budget is exceeded.

Bus and interrupt budget

Best for: Real-time responsiveness and protocol capacity

  • Method: Estimate transfer time per message, multiply by the message rate and compare the result with the bus capacity and the CPU time spent in ISRs.
  • Precision: Include framing bits, acknowledgements, clock stretching and software overhead, not only payload bytes.
  • Safeguards: Keep CAN utilization below roughly 60-70% and measure interrupt latency under worst-case load.

Quick Reference Tables

Widths, ranges, boundary values and language types — the lookup tables you will come back to.

Embedded bus and timing quick reference

InterfaceTypical rateEffective payloadWatch for
UART 115200 8N110 bits per byteAbout 11.5 kB/sClock error, framing and flow control
I2C standard mode100 kHzAbout 10-12 kB/sClock stretching and bus capacitance
I2C fast mode400 kHzAbout 40-50 kB/sRise time and pull-up sizing
SPI10 MHzAbout 1.25 MB/sMode, chip-select setup and trace length
CAN classical500 kbit/sKeep load below 60-70%Arbitration, jitter and message priority
RTOS tick1 ms1,000 interrupts/sISR duration and scheduler overhead

Formulas & How to Use This Calculator

Core relationship

f_timer = clock / (prescaler x (ARR + 1)); period = 1 / f_timer

Unit convention

Inputs are timer clock (hz), target frequency (hz), counter width, auto-reload value (0 = auto), maximum prescaler. Keep each unit consistent with the result rows; convert before comparing estimates that use different units or time windows.

Validation

1. Reproduce the headline from the displayed rows. 2. Compare the result with a measured, billed or benchmarked value. 3. Re-run with a boundary input and with a scaled input. 4. Record the assumptions, source and date next to the result.

How to use — step by step

  1. Open the Timer Prescaler Calculator and review the input defaults.
  2. Replace timer clock (hz) with your measured or planned value.
  3. Set target frequency (hz) (Desired update or interrupt rate.).
  4. Set counter width (default 16).
  5. Set auto-reload value (0 = auto) (Set a known ARR to check it, or leave 0 to choose one automatically.).
  6. Read the headline result and compare it with the supporting rows in the breakdown.
  7. Test one boundary value and one scaled value to understand the sensitivity.
  8. Copy or record the result together with the units and assumptions used.

Worked Examples

Real numbers and raw bits, fully worked — copy the pattern for your own code or protocol.

Default scenario

Inputs

Timer clock (Hz) = 84000000; Target frequency (Hz) = 1000; Counter width = 16; Auto-reload value (0 = auto) = 0; Maximum prescaler = 65536

Math

f_timer = clock / (prescaler x (ARR + 1)); period = 1 / f_timer

Result

PSC 2, ARR 41,999 · Timer clock: 84 MHz · Target frequency: 1,000 Hz · Counter width: 16 bits

Scaled scenario

Inputs

Timer clock (Hz) = 168000000; Target frequency (Hz) = 1500; Counter width = 16; Auto-reload value (0 = auto) = 0; Maximum prescaler = 65536

Math

f_timer = clock / (prescaler x (ARR + 1)); period = 1 / f_timer

Result

PSC 2, ARR 55,999 · Timer clock: 168 MHz · Target frequency: 1,500 Hz · Counter width: 16 bits

Advanced Guide: Timer Prescaler Calculator

Beyond the calculator — how to use timer prescaler calculator in real engineering, capacity and review decisions.

What the Timer Prescaler Calculator measures

Finds a prescaler and auto-reload pair for a target timer frequency, with resolution, error and register-range checks. It turns ordinary project, infrastructure or runtime inputs into a visible number you can defend in a design review. The calculator is deliberately transparent: the result is derived from f_timer = clock / (prescaler x (arr + 1)); period = 1 / f_timer rather than from an opaque benchmark, so you can change one assumption and immediately see how the answer moves.

The formula and its units

The core relationship is f_timer = clock / (prescaler x (ARR + 1)); period = 1 / f_timer. Every field is labelled with its unit and expected range next to the input, and the breakdown repeats the intermediate values so a reviewer can verify the arithmetic. When you compare two results, make sure both were computed with the same units and the same time window; most apparent differences in embedded programming & low-level computing estimates are really unit or scope mismatches.

A worked check with the default inputs

With the default inputs (Timer clock (Hz) = 84000000; Target frequency (Hz) = 1000; Counter width = 16; Auto-reload value (0 = auto) = 0; Maximum prescaler = 65536), the calculator reports PSC 2, ARR 41,999. Change Timer clock (Hz) and the headline moves to PSC 2, ARR 55,999. That sensitivity is useful: if a small input change produces a large output change, the assumption deserves a source or a range rather than a single guess.

Inputs, assumptions and defaults

The inputs are Timer clock (Hz) (Input clock to the timer counter.); Target frequency (Hz) (Desired update or interrupt rate.); Counter width (default 16); Auto-reload value (0 = auto) (Set a known ARR to check it, or leave 0 to choose one automatically.); Maximum prescaler (Hardware limit for the prescaler register.). Defaults are realistic starting points, not recommendations for your system. Replace them with measured values wherever possible: production metrics, billing exports, compiler sizeof output, provider documentation or historical project data. Keep the assumptions beside the result so the estimate can be reproduced later.

Reading a low or high result

A low result usually means small scale, generous headroom or optimistic assumptions; a high result means the opposite. Neither is automatically good or bad. Ask which input dominates the change, then compare the result with a second method such as a benchmark, a bill, a query plan or a pilot. Agreement between two independent methods is much stronger evidence than a single formula.

Common mistakes in embedded programming & low-level computing

Forgetting the +1 in the ARR divider. Choosing a prescaler that makes ARR exceed the counter width. Document the units, the measurement window and the scope, and re-run the calculation when the workload or architecture changes.

How to validate the result

Validate with three checks: reproduce the headline from the displayed rows, compare the estimate with a measured production value, and test a boundary case (zero, one, the maximum or a doubled input). If the three checks disagree, the model or the input is wrong. Record the validation date and the data source so the next reviewer can repeat it.

Where this fits in Embedded Programming & Low-Level Computing

This tool supports registers, GPIO masks, firmware and RAM budgets, UART, SPI, I2C, CAN timing, interrupts and timer prescalers. Use it during design, capacity review, incident follow-up or cost planning. Pair it with the related calculators to cover adjacent assumptions, and keep the output with the design decision so future changes can be compared against a known baseline.

Limitations and precision

No estimate is more accurate than its inputs. The calculator assumes steady-state behavior, consistent units and the defaults described in the field help. Real systems have bursty traffic, fragmentation, retries, background jobs and measurement error. Where the result informs a commitment or a budget, add a range and a contingency rather than treating the headline as a guarantee.

Related calculations to run next

The most useful companion tools are Interrupt Frequency Calculator, CPU Clock Cycle Converter, Instruction Cycle Time Calculator, UART Baud Rate Error Calculator. Together they cover the neighbouring assumptions — for example scale, overhead, failure headroom and cost — so you can present a complete estimate rather than a single isolated number.

Language & Platform Playbooks

The same bit pattern behaves differently in every language. Here is how to adapt the conversion safely.

STM32 HAL

Use SystemCoreClock, the reference manual clock tree and the UART BRR or timer PSC/ARR registers. Confirm timing with the DWT cycle counter, a GPIO toggle and the linker map file. For timer prescaler calculator, use the same units and time window described in the calculator so the result lines up with your platform data.

ESP32 / Arduino

Use micros() for coarse timing, ledc for PWM, and the FreeRTOS tick for scheduling. Check the partition table and the dual-core interrupt affinity when measuring real-time behavior. For timer prescaler calculator, use the same units and time window described in the calculator so the result lines up with your platform data.

AVR / 8-bit MCUs

Compute UBRR from F_CPU and the baud rate, keep ISRs short, and store constant tables in PROGMEM. Watch the small RAM budget and the limited stack. For timer prescaler calculator, use the same units and time window described in the calculator so the result lines up with your platform data.

Zephyr / FreeRTOS

Set CONFIG_SYS_CLOCK_TICKS_PER_SEC or configTICK_RATE_HZ deliberately, assign interrupt priorities, and enable stack watermarking or runtime statistics before tuning task sizes. For timer prescaler calculator, use the same units and time window described in the calculator so the result lines up with your platform data.

Glossary: Key Terms Explained

Prescaler
A divider between a clock source and a timer or peripheral that sets the counting rate.
ISR
An interrupt service routine, the function that runs in response to a hardware or software interrupt.
Baud rate
The number of signal symbols transmitted per second; for UART 8N1 it is close to the byte rate divided by ten.
Duty cycle
The fraction of one period during which a digital signal is high, usually expressed as a percentage.
Stack watermark
The deepest address a stack has reached, used to measure worst-case stack usage.
Memory-mapped I/O
A scheme where peripheral registers appear at ordinary memory addresses and are accessed with normal loads and stores.

Common Mistakes & Pro Tips

Mistakes that break conversions

  • Forgetting the +1 in the ARR divider.
  • Choosing a prescaler that makes ARR exceed the counter width.
  • Mixing units or time windows when comparing two timer prescaler calculator results.
  • Treating the default values as a recommendation instead of a starting point.
  • Reporting a single number without the assumptions, range or measurement date behind it.

Pro tips for production code

  • Check the timer clock after every PLL or bus-prescaler change.
  • Use a 32-bit timer when you need both long periods and fine resolution.
  • Measure the dominant input before refining the model; the largest uncertainty deserves the most attention.
  • Keep a short record of the inputs, result and date so future comparisons are meaningful.
  • Validate the estimate with a second method — a benchmark, a bill or production telemetry.
  • Pair this result with the related calculators (Interrupt Frequency Calculator, CPU Clock Cycle Converter, Instruction Cycle Time Calculator) for a complete picture.

Who Should Use This Calculator?

  • ▸ Configuring a 1 kHz timer interrupt.
  • ▸ Generating a precise PWM period.
  • ▸ Checking whether a timer can reach a slow update rate.
  • ▸ Capacity review: turn timer clock (hz) into a defensible planning number.
  • ▸ Design review: compare two architectures or configurations with the same assumptions.
  • ▸ Incident follow-up: quantify the gap between the estimate and observed behaviour.

Related Programming Calculators

Keep going — every tool below links directly into this workflow.

People Also Search For

Tap a long-tail question to jump to the matching FAQ answer.

Frequently Asked Questions (28)

Long-tail answers: each question targets a real Google search about timer prescaler calculator — click to expand.

Showing 28 of 28 answers.

How do I choose a timer prescaler and ARR?
Compute the ideal total divider as timer clock / target frequency. Choose a prescaler that keeps ARR within the counter width, then set ARR = round(ideal divider / prescaler) - 1. Verify the actual frequency and error.
Should I use a large prescaler or a large ARR?
Prefer the largest prescaler that still leaves enough ARR range for the period you need. A larger prescaler reduces the counter resolution, so use a smaller prescaler when fine duty-cycle or pulse-width control matters.
What does the Timer Prescaler Calculator calculate?
Finds a prescaler and auto-reload pair for a target timer frequency, with resolution, error and register-range checks. The output includes the headline value, the intermediate rows and the formula, so you can audit the result rather than accepting a black-box number.
What formula does the Timer Prescaler Calculator use?
It uses f_timer = clock / (prescaler x (ARR + 1)); period = 1 / f_timer. The calculator evaluates the formula with the values shown in the input panel and displays each intermediate term in the breakdown.
What should I enter for timer clock (hz)?
Input clock to the timer counter. The default is 84000000; replace it with a measured value whenever one is available, and keep the same unit for every comparison.
What should I enter for target frequency (hz)?
Desired update or interrupt rate. The default is 1000; replace it with a measured value whenever one is available, and keep the same unit for every comparison.
What should I enter for counter width?
Enter counter width in the stated unit. The default is 16; replace it with a measured value whenever one is available, and keep the same unit for every comparison.
What should I enter for auto-reload value (0 = auto)?
Set a known ARR to check it, or leave 0 to choose one automatically. The default is 0; replace it with a measured value whenever one is available, and keep the same unit for every comparison.
What should I enter for maximum prescaler?
Hardware limit for the prescaler register. The default is 65536; replace it with a measured value whenever one is available, and keep the same unit for every comparison.
How do I interpret the Timer Prescaler Calculator result?
The headline is PSC 2, ARR 41,999. PSC 2, ARR 41,999, 1,000 Hz (0% error). A single number is less useful than the trend and the assumptions behind it.
What units does the Timer Prescaler Calculator use?
The inputs are Timer clock (Hz), Target frequency (Hz), Counter width, Auto-reload value (0 = auto), Maximum prescaler. Units are shown on every field and repeated in the result rows. Do not mix decimal and binary units or seconds and milliseconds without converting first.
What are common mistakes when calculating timer prescaler calculator?
Forgetting the +1 in the ARR divider. Choosing a prescaler that makes ARR exceed the counter width. Re-run the calculation whenever the workload, architecture or pricing changes.
When should I use the Timer Prescaler Calculator?
Configuring a 1 kHz timer interrupt. Generating a precise PWM period. Checking whether a timer can reach a slow update rate. It is a client-side planning tool, not a substitute for production monitoring or a benchmark.
How can I validate the Timer Prescaler Calculator result?
Reproduce the headline from the displayed rows, compare it with a measured or billed value, and test a boundary input such as zero, one or double the normal value. If the checks disagree, correct the input or the model before using the result.
What is a worked example of the Timer Prescaler Calculator?
For Timer clock (Hz) = 84000000; Target frequency (Hz) = 1000; Counter width = 16; Auto-reload value (0 = auto) = 0; Maximum prescaler = 65536, the result is PSC 2, ARR 41,999 (Timer clock: 84 MHz; Target frequency: 1,000 Hz).
What related calculators should I use with the Timer Prescaler Calculator?
Use Interrupt Frequency Calculator, CPU Clock Cycle Converter, Instruction Cycle Time Calculator, UART Baud Rate Error Calculator to cover the adjacent assumptions. Running several related estimates is more reliable than over-tuning one number.
Is the Timer Prescaler Calculator free and private?
Yes. The calculator runs entirely in your browser, requires no sign-up and sends no inputs to a server. That makes it safe for approximate capacity planning, although production secrets and customer data should never be pasted into any web page.
Can I use the Timer Prescaler Calculator for production planning?
Use it as a first-pass model and confirm the result with production measurements, provider documentation or a staging benchmark. Add headroom for uncertainty, and record the date and data source so the estimate can be reviewed later.
How sensitive is the Timer Prescaler Calculator to timer clock (hz)?
With the default value the result is PSC 2, ARR 41,999; when timer clock (hz) is changed to 168000000, the result becomes PSC 2, ARR 55,999. Use this comparison to identify which input deserves the most careful measurement.
How accurate does a UART baud rate need to be?
Keep the total clock error below about 2-3% so the receiver samples each bit near its center. Both ends contribute error, so budget the oscillator tolerance and the divider rounding together. A 2% error at 115200 baud is roughly 2300 baud, which is already close to the limit for a 10-bit frame.
How much stack does an embedded task need?
Start from the deepest call chain, add the largest interrupt nesting frame and the context-save area, then measure the high-water mark under realistic load. Add at least 20-30% headroom for compiler changes, logging and rare error paths; a stack overflow in firmware usually corrupts adjacent memory instead of producing a clean exception.
Why keep CAN bus utilization below 70%?
CAN uses priority-based arbitration, so a busy bus increases the latency of low-priority frames and makes worst-case response time hard to bound. Staying below roughly 60-70% leaves room for retransmissions, error frames and occasional bursts without missing deadlines.
How do I choose a timer prescaler and reload value?
The update rate is f_clock / ((prescaler + 1) × (reload + 1)). Choose the largest prescaler that still gives the resolution you need, then set the reload value for the desired period. Prefer a reload value that fits the timer width and verify the first tick against a scope or the DWT cycle counter.
What does a typical Timer Prescaler Calculator output look like?
The default scenario shows PSC 2, ARR 41,999 with rows for timer clock, target frequency, counter width. The exact wording of each row is shown in the technical breakdown above.
How precise is the Timer Prescaler Calculator?
The arithmetic is exact for the values entered, but the result is only as good as the inputs. Engineering estimates typically carry ±10–30% uncertainty, and capacity models can be further off when traffic is bursty. Report a range and the main assumptions rather than a single deceptively precise number.
Does the Timer Prescaler Calculator include overhead?
It includes the overhead represented by its input fields: timer clock (hz), target frequency (hz), counter width, auto-reload value (0 = auto), maximum prescaler. If your system has additional overhead — retries, replication, fragmentation, taxes or contingency — add it explicitly and document the adjustment.
How often should I re-run the Timer Prescaler Calculator?
Re-run it when the workload changes materially, when a provider or platform changes its limits or pricing, and at least once per planning cycle. Keep each result with the date and inputs so you can compare the trend instead of only the latest snapshot.
What is the best first step after using the Timer Prescaler Calculator?
Write down the three assumptions that most affect the result, then verify the largest one with a measurement. For embedded programming & low-level computing, that usually means checking a production metric, a bill or a documented limit before changing configuration.

Resources & Authoritative References

Further reading

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