AN2260 datasheet(7/10 Pages) STMICROELECTRONICS

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AN2260

2 UART SystemMemory boot mode

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2.2

Choosing the UART baud rate

The calculation of the serial baud rate for UART0, from the length of the first zero byte that is

received, allows the operation of the STR73x SystemMemory boot loader within a wide range

of baud rates. However, the upper and lower limits have to be kept, in order to insure proper

data transfer.

STR73x Baud Rate = Fcpu / 16 * UART0_BAUDRATE

The STR73x uses Timer Base 0 to measure the length of the initial zero byte. The quantization

uncertainty of this measurement implies the first deviation from the theoretical baud rate. The

next deviation is implied by the computation of the UART0_BAUDRATE reload value from the

timer contents. The formula below shows the association:

TB0_CNT / 9 = 1 Bit Time in Fcpu (9 Bit Times the first ZERO Byte)

UART0_BAUDRATE = TB0_CNT / 9 / 16 = TB0_CNT / 144

For a correct data transfer from the host to the STR73x, the maximum deviation between the

internal initialized baud rate for UART0 and the real baud rate of the host should be below

2.5%. The deviation (fB, in percent) between the host baud rate and STR73x baud rate can be

calculated via the formula below:

fB= abs( (STR73x Baud Rate - Host Baud Rate) / STR73x Baud Rate) * 100%

fB<= 2.5%

Note:

Function (fB) does not consider the tolerances of oscillators and other devices supporting the

serial communication.

This baud rate deviation is a nonlinear function depending on the CPU clock and the baud rate

of the host. The maximum of the function (fB) increases with the host baud rate. This is due to

the smaller baud rate pre-scale factors, and the implied higher quantization error.

2.2.1

Minimum baud rate

BLow is the minimum baud rate determined by the maximum count capacity of Timer Base 0

when measuring the zero byte. This will be directly related to system clock frequency. Using the

maximum TB0_CNT value of 65535 in the equations above we end up with a minimum Baud

Rate of 1648 at Fcpu = 12MHz. The lowest standard Baud Rate for this case would be 2400.

Baud rates below BLow would cause TB0 to overflow. If this occurs, it would not be possible to

properly initialize the UART0.

2.2.2

Maximum baud rate

BHigh is the highest baud rate in which the deviation still does not exceed the limit. All baud

rates between BLow and BHigh are below the deviation limit.

2.2.3

Higher baud rate deviations

A baud rate may be used as long as the actual deviation does not exceed the limit. Certain

lower baud rates may violate the deviation limit, while an even higher baud rate stays very well

below it. This relationship depends on the host interface.

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