Documentation sections

SPI Timing & Signal Integrity Calculator

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

Slug spi-timing. Calculate SPI bus timing parameters including bit period, frame time, maximum clock frequency limited by trace capacitance, and signal slew rate

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 spi-timing
Name	Unit	Range	Default	Description
clockFreqnumber	MHz	at least 0.001	10	SPI SCLK frequency in MHz
dataWidthnumber	bits	1 … 32	8	Number of data bits per SPI frame
traceCapacitancenumber	pF	at least 0.1	20	Estimated total capacitive load on the signal line (PCB trace + receiver)
driveStrengthnumber	mA	at least 0.1	4	GPIO output drive current capability in mA

Outputs

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

Outputs of spi-timing
Field	Unit	Description
bitPeriod	ns	Bit Period
framePeriod	μs	Frame Period
maxFreqCapacitiveheadline	MHz	Max Freq (RC-limited)
slewRate	V/ns	Signal Slew Rate
dataRate	Mbps	Data Rate
riseTime	ns	2.2τ, the 10–90 % rise of the RC charge through the calculator's drive model, R = 3.3 V / I_drive (825 Ω at 4 mA); a CMOS output is usually stiffer than that (V_OL / I_OL), so this edge is a slow bound
edgeShare_pct	%	How much of the half period the edge takes before the sampling edge; 100 % at the RC-limited maximum

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": "spi-timing",
    "inputs": {
      "clockFreq": 10,
      "dataWidth": 8,
      "traceCapacitance": 20,
      "driveStrength": 4
    }
  }'

import rftools

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

result = client.calculate("spi-timing", {
    "clockFreq": 10,
    "dataWidth": 8,
    "traceCapacitance": 20,
    "driveStrength": 4,
})
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": "spi-timing",
    "inputs": {
      "clockFreq": 10,
      "dataWidth": 8,
      "traceCapacitance": 20,
      "driveStrength": 4
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "spi-timing",
	  "inputs": {
	    "clockFreq": 10,
	    "dataWidth": 8,
	    "traceCapacitance": 20,
	    "driveStrength": 4
	  }
	}`)
	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": "spi-timing",
  "values": {
    "bitPeriod": 100,
    "framePeriod": 0.8,
    "maxFreqCapacitive": 13.774104683195596,
    "slewRate": 0.09090909090909091,
    "dataRate": 10,
    "riseTime": 36.3,
    "edgeShare_pct": 72.6
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:spi-timing",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "clockFreq": {
          "min": 0.001,
          "max": null,
          "unit": "MHz"
        },
        "dataWidth": {
          "min": 1,
          "max": 32,
          "unit": "bits"
        },
        "traceCapacitance": {
          "min": 0.1,
          "max": null,
          "unit": "pF"
        },
        "driveStrength": {
          "min": 0.1,
          "max": null,
          "unit": "mA"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "clockFreq": 10,
      "dataWidth": 8,
      "traceCapacitance": 20,
      "driveStrength": 4
    },
    "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 pageSPI Timing & Signal Integrity Calculator, at these values