HT45B0F

Rev. 1.00

12

June 7, 2011

Baud Rate Generator

To setup the speed of the serial data communication,

the UART function contains its own dedicated baud rate

generator. The baud rate is controlled by its own internal

free running 8-bit timer, the period of which is deter-

mined by two factors. The first of these is the value

placed in the baud rate register BRG and the second is

the value of the BRGH bit with the control register

UCR2. The BRGH bit decides if the baud rate generator

is to be used in a high speed mode or low speed mode,

which in turn determines the formula that is used to cal-

culate the baud rate. The value N in the BRG register

which is used in the following baud rate calculation for-

mula determines the division factor. Note that N is the

decimal value placed in the BRG register and has a

range of between 0 and 255.

UCR2 BRGH Bit

0

1

Baud Rate (BR)

f

[64 (N + 1)]

CLKI

f

[16 (N + 1)]

CLKI

By programming the BRGH bit which allows selection of

the related formula and programming the required value

in the BRG register, the required baud rate can be setup.

Note that because the actual baud rate is determined

using a discrete value, N, placed in the BRG register,

there will be an error associated between the actual and

requested value. The following example shows how the

BRG register value N and the error value can be calcu-

lated.

For a clock frequency of 4MHz, and with BRGH set to

²0² determine the BRG register value N, the actual

baud rate and the error value for a desired baud rate

of 4800.

From the above table the desired baud rate BR =

f

[64 (N + 1)]

CLKI

Re-arranging this equation gives N =

f

(BRx64)

CLKI

- 1

Giving a value for N =

4000000

4800 64

()

x

- 1 = 12.0208

To obtain the closest value, a decimal value of 12 should

be placed into the BRG register. This gives an actual or

calculated baud rate value of BR=

4000000

[64(12 + 1)]

= 4808

Therefore the error is equal to

= 0.16%

4 8 0 8

4 8 0 0

The following tables show the actual values of baud rate and error values for the two value of BRGH.

Baud

Rate

K/BPS

Baud Rates for BRGH=0

BRG

Kbaud

Error (%)

BRG

Kbaud

Error (%)

BRG

Kbaud

Error (%)

0.3

207

0.300

0.16

185

0.300

0.00

¾¾¾

1.2

51

1.202

0.16

46

1.190

-0.83

92

1.203

0.23

2.4

25

2.404

0.16

22

2.432

1.32

46

2.380

-0.83

4.8

12

4.808

0.16

11

4.661

-2.90

22

4.863

1.32

9.6

6

8.929

-6.99

5

9.321

-2.90

11

9.322

-2.90

19.2

2

20.833

8.51

2

18.643

-2.90

5

18.643

-2.90

38.4

¾¾¾¾¾¾

2

32.286

-2.90

57.6

0

62.500

8.51

0

55.930

-2.90

1

55.930

-2.90

115.2

¾¾¾¾¾¾

0

111.859

-2.90

Baud Rates and Error Values for BRGH

= 0