How Do I Set the Baud-Rate for the 8051’s Dedicated UART?

The Triscend E5 Configurable System-on-Chip (CSoC) device contains an embedded, accelerated 8051 microcontroller. The E5’s dedicated UART is directly compatible with the UART on the original 8051.

One common question is how to set up the dedicated UART’s baud-rate generator. In modes 1 and 3, where the UART has an adjustable, variable baud rate, the UART uses either the 8051’s dedicated Timer 1 or Timer 2 as the baud-rate generator. The baud is determined by a few factors including …

· the bus clock operating frequency, F_BUSCLK

· the desired baud rate, BAUD

· the baud-rate generator divisor value, DIVISOR

· the acceptable baud-rate error, %ERROR

· The availability of either the 8051’s Timer 1 or Timer 2 dedicated timer/counter

Timer 1 as the Baud-Rate Generator

When using Timer 1 for the baud-rate generator, the 8-bit TH1 register stores the DIVISOR value. The maximum value for DIVISOR ≤ 255.

Calculate the appropriate DIVISOR value using the following equation. Round the result up or down to the closest integer.

The actual baud rate generated using the DIVISOR value may be different than the desired baud rate, because DIVISOR is rounded to the nearest integer value.

The operating frequency and the integer value for DIVISOR may not provide the precise desired baud rate. Because each bit of the UART is oversampled and each data transaction re-synchronized, the UART tolerates slight errors in the baud rate, safely up to 3%. Calculate the percent baud-rate error using the following equation.

Timer 2 as the Baud-Rate Generator

When using Timer 2 for the baud-rate generator, the 16-bit RCAP2 register stores the DIVISOR value. The maximum value for DIVISOR ≤ 65,535. Because Timer 2 has a 16-bit DIVISOR, it generates a wider variety of baud rates than Timer 1.

Calculate the appropriate DIVISOR value using the following equation. Round the result up or down to the closest integer.

The 16-bit DIVISOR values is stored in a 16-bit register called RCAP2, which consists of two 8-bit registers called RCAP2H and RCAP2L RCAP2H is the most-significant byte and RCAP2L is the least-significant byte.

The actual baud rate generated using the DIVISOR value may be different than the desired baud rate, because DIVISOR is rounded to the nearest integer value.

8051 UART “Magic” Frequencies

Because of the UART hardware implementation, there are a few “magic” clock frequencies, F_BUSCLK, that provide precise baud rate outputs—i.e. zero error—for common baud rates such as 4,800 bps, 9,600 bps, etc.

These frequencies are any multiple of 1.8432 MHz, while the result is less than 40 MHz.

If you desire a precise baud rate, with zero error, and you have some flexibility in your operating frequency, the following tables includes some of the more commonly available crystal oscillators that operate at a “magic” frequency.

Commonly Available Crystal Oscillators at
the “Magic” Frequency

n	Frequency
2	3.6864 MHz
6	11.0592 MHz
10	18.432 MHz
12	22.1184 MHz
14	25.8048 MHz
16	29.4912 MHz
20	36.864 MHz

Using the 8051 UART Baud-Rate Calculations Spreadsheet

The Excel spreadsheet associated with this article quickly calculates the DIVISOR for a given set of operating conditions.

Example:

What is the DIVISOR value when operating at 25 MHz and the application requires a UART with a 38,400 bits per second baud rate? Should the application use Timer 1 or Timer 2 as the baud-rate generator?

Open the spreadsheet, and specify the E5’s operating clock frequency, which is 25 MHz in this example, or 25,000,000.

Specify the desired baud, 38,400 bits per second. The spreadsheet immediately calculates available solutions. For the spreadsheet calculations, any answer that results in 3% error or less is considered a valid answer.

Under the given operating conditions, there is no solution using Timer 1, regardless if the SMOD bit.

There is only one valid solution, using Timer 2. With RCAP2H loaded with 255 and RCAP2L loaded with 236, the 8051 UART communicates at 39,062.5 bits per second, which is within 1.73% of the desired baud rate frequency.

Setting the Baud Rate Values in FastChip

After determining the baud rate values, you can either program the values directly via your application code or use FastChip’s graphical interface, as shown below.

With FastChip open, click the UART button in the Dedicated Resources section.

Choose the serial port operating mode. Mode 1 or mode 3 is for a variable baud rate.

The transmitter and receiver baud rates can be different. In this example, however, select Timer 2 as the baud-rate generator for both the transmitter and receiver.

Scroll down to Timer 2 Settings. The spreadsheet provides the DIVISOR values in decimal. By default, FastChip uses hexadecimal. Click the radix buttons next to the text boxes until the both display “Dec”.

Using the values from the spreadsheet, enter 255 for RCAP2H and 236 for RCAP2L.

Click OK. FastChip automatically creates an initialization routine based on your settings. Clicking Generate from the FastChip toolbar creates an initialization routine for your entire design. To view the initialization routine for the UART, click UART to open the dialog box and then click View Header.

FastChip Version: 2.2.0