Rev. 1.50
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Rev. 1.50
163
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HT66F0175/HT66F0185
A/D Flash MCU with EEPROM
HT66F0175/HT66F0185
A/D Flash MCU with EEPROM
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 counter, the
period of which is determined by two factors. The first of these is the value placed in the BRG
register and the second is the value of the BRGH bit within the UCR2 control register. 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 calculate the baud rate. The value in the BRG register,
N, which is used in the following baud rate calculation formula 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
Ba�d Rate (BR)
f
SYS
[64(N+1)]
f
SYS
[16(N+1)]
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 calculated.
BRG Register
Bit
7
6
5
4
3
2
1
0
Name
BRG�
BRG6
BRG5
BRG4
BRG3
BRG�
BRG1
BRG0
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
POR
x
x
x
x
x
x
x
x
“x”: �nknown
Bit 7~0
BRG7~BRG0
: Baud Rate values
By programming the BRGH bit in the UCR2 register which allows selection of the
related formula described above and programming the required value in the BRG
register, the required baud rate can be setup.
Calculating the Baud Rate and Error Values
For a clock frequency of 4MHz, and with BRGH set to 0 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
SYS
[64(N+1)]
Re-arranging this equation gives N =
f
SYS
(BR×64) - 1
Giving a value for N =
4000000
(4800×64) - 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 4808-4800
4800
= 0.16%