ST10F269Z2QX View Datasheet(PDF) - STMicroelectronics
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ST10F269Z2QX Datasheet PDF : 161 Pages
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ST10F269Z2Qx
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 ST10F269Z2Qx with a wide range of
Baud rates. However, the upper and lower limits
have to be kept, in order to insure proper data
transfer.
BST10F269Z2Qx
=
3----2-----×----(---S--f--C0----PB----U-R----L-----+-----1----)
The ST10F269Z2Qx 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:
S0BRL = T-----6--7--–-2----3---6--
, T6 = 94-- × B--f--C-H---P-o---U-s----t
For a correct data transfer from the host to the
ST10F269Z2Qx 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 ST10F269Z2Qx Baud rate can be
calculated via the formula below:
FB = -B----C-----o--B-n----Ct--r---o-–--n--B--t--rH-----o----s----t × 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 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.
Figure 8 : Baud Rate Deviation Between Host and ST10F269Z2Qx
I
FB
2.5%
BLow
BHigh
II
BHOST
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