ST10F269ZX datasheet(34/184 Pages) STMICROELECTRONICS

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5 - INTERNAL FLASH MEMORY

ST10F269

34/184

5.6.5 - Choosing the Baud Rate for the BSL

The calculation of the serial Baud rate for ASC0

from the length of the first zero Byte that is

received, allows the operation of the bootstrap

loader of the ST10F269 with a wide range of Baud

rates. However, the upper and lower limits have to

be kept, in order to insure proper data transfer.

The ST10F269 uses timer T6 to measure the

length of the initial zero Byte. The quantization

uncertainty of this measurement implies the first

deviation from the real Baud rate, the next

deviation is implied by the computation of the

S0BRL reload value from the timer contents. The

formula below shows the association:

For a correct data transfer from the host to the

ST10F269 the maximum deviation between the

internal initialized Baud rate for ASC0 and the real

Baud rate of the host should be below 2.5%. The

deviation (FB, in percent) between host Baud rate

and ST10F269 Baud rate can be calculated via

the formula below:

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 maxima of the function (FB)

increase with the host Baud rate due to the

smaller Baud rate pre-scaler factors and the

implied higher quantization error (see Figure 8).

The minimum Baud rate (BLow in the Figure 8) is

determined by the maximum count capacity of

timer T6, when measuring the zero Byte, and it

depends on the CPU clock. Using the maximum

T6 count 216 in the formula the minimum Baud

rate can be calculated. The lowest standard Baud

rate in this case would be 1200 Baud. Baud rates

below BLow would cause T6 to overflow. In this

case ASC0 cannot be initialized properly.

The maximum Baud rate (BHigh in the Figure 8)

is the highest Baud rate where the deviation still

does not exceed the limit, so all Baud rates

between BLow and BHigh are below the deviation

limit. The maximum standard Baud rate that fulfills

this requirement is 19200 Baud.

Higher Baud rates, however, may be used as

long as the actual deviation does not exceed the

limit. A certain Baud rate (marked ’I’ in Figure 8)

may violate the deviation limit, while an even

higher Baud rate (marked ’II’ in Figure 8) stays

very well below it. This depends on the host

interface.

CPU

S0BRL

()

------------------------------------------------

BST10F269 =

S0BRL

–

--------------------

---

CPU

Host

-----------------

C ontr

Ho st

–

C ontr

--------------------------------------------

100

% ,

2.5

≤

Figure 8 : Baud Rate Deviation Between Host and ST10F269

BLow

2.5%

BHigh

BHOST

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