Documentation sections

CAN Bus Bit Timing Calculator

POSThttps://rftools.io/api/py/v1/calculate

Slug can-bus-timing. Calculate CAN bus bit timing parameters including prescaler, time quanta, sync segment, propagation segment, and phase buffer segments for a given baud rate and sample point

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of can-bus-timing
Name	Unit	Range	Default	Description
clockFreqnumber	MHz	at least 1	48	Microcontroller peripheral clock frequency feeding the CAN controller
baudRatenumber	kbps	at least 1	500	Target CAN bus baud rate
samplePointnumber	%	50 … 95	87.5	Sample point position as percentage of bit time (CiA 601 recommends 75–87.5%)
sjwnumber	tq	1 … 4	1	How far a resynchronisation may move the sample point; a wider jump lets the bus take a looser clock, up to the length of either phase segment

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of can-bus-timing
Field	Unit	Description
tq	ns	Time Quantum (TQ)
nominalBitTime	tq	Nominal Bit Time
syncSeg	tq	Sync Segment
propSeg	tq	Propagation Segment
phase1	tq	Phase Buffer 1 (BS1)
phase2	tq	Phase Buffer 2 (BS2)
prescalerheadline	none	Prescaler
bitRateActual	kbps	The clock over the prescaler and the quanta of the bit as programmed
bitRateError_pct	%	Actual over requested, signed; it adds to the oscillator's own tolerance
samplePointActual	%	Sync, propagation and phase 1 over the whole bit, in whole quanta
oscTolerance_pct	%	The largest deviation from nominal each node's clock may have (ISO 11898-1): the smaller of min(PS1, PS2)/(2·(13·NBT − PS2)) and SJW/(20·NBT)

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "can-bus-timing",
    "inputs": {
      "clockFreq": 48,
      "baudRate": 500,
      "samplePoint": 87.5,
      "sjw": 1
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("can-bus-timing", {"clockFreq": 48, "baudRate": 500, "samplePoint": 87.5, "sjw": 1})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "can-bus-timing",
    "inputs": {
      "clockFreq": 48,
      "baudRate": 500,
      "samplePoint": 87.5,
      "sjw": 1
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "can-bus-timing",
	  "inputs": {
	    "clockFreq": 48,
	    "baudRate": 500,
	    "samplePoint": 87.5,
	    "sjw": 1
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "can-bus-timing",
  "values": {
    "tq": 125.00000000000001,
    "nominalBitTime": 16,
    "syncSeg": 1,
    "propSeg": 6,
    "phase1": 7,
    "phase2": 2,
    "prescaler": 6,
    "bitRateActual": 500,
    "bitRateError_pct": 0,
    "samplePointActual": 87.5,
    "oscTolerance_pct": 0.3125
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:can-bus-timing",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "clockFreq": {
          "min": 1,
          "max": null,
          "unit": "MHz"
        },
        "baudRate": {
          "min": 1,
          "max": null,
          "unit": "kbps"
        },
        "samplePoint": {
          "min": 50,
          "max": 95,
          "unit": "%"
        },
        "sjw": {
          "min": 1,
          "max": 4,
          "unit": "tq"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "clockFreq": 48,
      "baudRate": 500,
      "samplePoint": 87.5,
      "sjw": 1
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "clockFreq"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pageCAN Bus Bit Timing Calculator, at these values