Baud Rate Calculation in 8051

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Baud Rate Calculator for 8051 Microcontrollers

Mode 1 (8-bit auto-reload) Mode 2 (8-bit auto-reload with TLx as divisor)
This is the value of the SCx bits in the TMOD register. For baud rate generation using Timer 1, SCx bits are typically 0 for Mode 2.

Understanding Baud Rate Calculation in 8051 Microcontrollers

The baud rate is a fundamental concept in serial communication, representing the number of signal or symbol changes that occur per second. In the context of the 8051 microcontroller, accurately calculating and setting the baud rate is crucial for establishing reliable communication with other devices, such as computers, sensors, or other microcontrollers. The 8051 utilizes its built-in serial port (UART) and timers to generate the required baud rate.

How Baud Rate is Generated in 8051

The 8051 microcontroller typically uses Timer 1 to generate the baud rate for its serial port. The process involves configuring Timer 1 in a specific mode and setting its reload value based on the system's oscillator frequency.

Timer Modes for Baud Rate Generation:

  • Mode 1 (8-bit auto-reload): In this mode, Timer 1 operates as an 8-bit timer with a 16-bit auto-reload value. The TH1 register holds the high byte of the reload value, and TL1 holds the low byte. For baud rate generation, Timer 1 is typically set to run in Mode 1, and the serial port control register (SCON) must be configured appropriately (SM0=0, SM1=1).
  • Mode 2 (8-bit auto-reload with TLx as divisor): This mode is specifically designed for baud rate generation. In Mode 2, Timer 1 is configured as an 8-bit timer, and TL1 automatically reloads the value from TH1 whenever Timer 1 overflows. The SCx bits in the TMOD register (where x is the timer number) determine the prescaler value. For baud rate generation using Timer 1, the SCx bits are usually set to 0, meaning no additional prescaling is applied by the timer itself before it starts counting. The baud rate is then determined by the value loaded into TH1.

The Calculation Formula

The baud rate is determined by the following formula:

Baud Rate = Oscillator Frequency / (Prescaler Value * N)

Where:

  • Oscillator Frequency: The crystal frequency of the 8051 microcontroller.
  • Prescaler Value: For Timer 1 in Mode 2, this is typically 32. This is because the timer counts up to the value in TH1, and then the serial port generates a start bit, 8 data bits, and a stop bit, totaling 10 bits per character. The formula implicitly accounts for this by dividing by 32 (which is 2 * 16, where 16 is related to the bit rate in Mode 2 and 2 is the basic timer clock division).
  • N: For Timer 1 in Mode 1, N is 16. For Timer 1 in Mode 2 (which is the standard for baud rate generation), N is effectively handled by the auto-reload mechanism and the bit structure of serial transmission. The common formula used for Mode 2 with Timer 1 is: Baud Rate = Oscillator Frequency / (32 * (256 – TH1)), where TH1 is the value loaded into the TH1 register.

However, a more direct way to calculate the baud rate given the oscillator frequency and timer mode is by understanding the required reload value for TH1:

TH1 Value = 256 – (Oscillator Frequency / (32 * Desired Baud Rate))

This calculator simplifies the process by allowing you to input the oscillator frequency and select the timer mode. It then calculates the resulting baud rate. If you know your desired baud rate, you can use the above formula to find the appropriate TH1 value to program your 8051.

Example Calculation:

Let's assume:

  • Oscillator Frequency = 11.0592 MHz (or 11,059,200 Hz)
  • Timer Mode = Mode 2 (standard for baud rate generation)
  • Prescaler Value = 1 (as SCx bits in TMOD are 0 for Timer 1 Mode 2)

Using the calculator:

The calculator will use the formula derived from Timer 1 Mode 2 operation: Baud Rate = Oscillator Frequency / (32 * (256 – TH1)). To find the baud rate when TH1 is not explicitly given but the timer is running in Mode 2, the formula effectively simplifies to relate directly to the oscillator frequency and a base factor for serial communication. A common scenario is to calculate the TH1 value for a specific baud rate. If we wanted to achieve a standard baud rate like 9600:

TH1 = 256 – (11059200 / (32 * 9600))

TH1 = 256 – (11059200 / 307200)

TH1 = 256 – 36

TH1 = 220

If TH1 = 220 is loaded, the baud rate will be approximately 9600.

This calculator focuses on deriving the baud rate from the oscillator frequency and mode.

function calculateBaudRate() { var oscFreq = parseFloat(document.getElementById("oscillatorFrequency").value); var timerMode = document.getElementById("timerMode").value; var prescalerValue = parseFloat(document.getElementById("prescalerValue").value); // This is usually fixed for baud rate gen in Mode 2 var resultDiv = document.getElementById("result"); resultDiv.innerHTML = ""; // Clear previous results if (isNaN(oscFreq) || oscFreq <= 0) { resultDiv.innerHTML = "Please enter a valid oscillator frequency (greater than 0)."; return; } var calculatedBaudRate = 0; var th1Value = ""; // To display the calculated TH1 value if (timerMode === "mode1") { // For Timer 1 Mode 1, baud rate generation is not automatic and requires external logic or specific configurations for fixed baud rates. // The standard formula for calculating TH1 for a desired baud rate is: // TH1 = 256 – (Oscillator Frequency / (12 * Desired Baud Rate)) // Since this calculator aims to show the baud rate from settings, and Mode 1 isn't direct for baud rate gen, we'll explain this limitation. resultDiv.innerHTML = "Note: Timer 1 in Mode 1 is not typically used for direct baud rate generation in the 8051. Mode 2 is the standard for this purpose."; return; } else if (timerMode === "mode2") { // Standard formula for Timer 1 Mode 2 baud rate generation: // Baud Rate = Oscillator Frequency / (32 * (256 – TH1)) // To find the baud rate, we need a TH1 value. The calculator will calculate the TH1 value needed for a common baud rate (e.g., 9600) // or, if we assume a specific TH1, calculate the baud rate. // A common approach in calculators is to find TH1 for a desired baud rate. // However, this calculator is designed to show the resulting baud rate *given* the frequency and mode. // In Mode 2, the implicit division factor related to the timer and serial port is 32. // The TH1 value determines the actual reload point. // Let's calculate the TH1 value for a common 9600 baud rate as an example if the oscillator is 11.0592MHz: var desiredBaudRate = 9600; // A common target var calculatedTH1 = 256 – (oscFreq / (32 * desiredBaudRate)); if (calculatedTH1 >= 0 && calculatedTH1 <= 255) { th1Value = Math.round(calculatedTH1).toString(16).toUpperCase(); if (th1Value.length < 2) { th1Value = "0" + th1Value; } // Recalculate baud rate using the determined TH1 for display accuracy calculatedBaudRate = oscFreq / (32 * (256 – Math.round(calculatedTH1))); resultDiv.innerHTML = "For a desired baud rate of " + desiredBaudRate + " Hz:"; resultDiv.innerHTML += "Calculated TH1 value (hex): " + th1Value + ""; resultDiv.innerHTML += "Calculated Baud Rate (approx): " + calculatedBaudRate.toFixed(2) + " Hz"; } else { resultDiv.innerHTML = "Could not calculate a valid TH1 value for a standard baud rate with the given oscillator frequency."; } } }

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