ST10F276E
Bootstrap loader
This process may go through several iterations or may directly execute the final application.
In all cases the ST10F276E still runs in BSL mode, that is, with the watchdog timer disabled
and limited access to the internal Flash area. All code fetches from the internal IFLASH area
(01’0000H...08’FFFFH) are redirected to the special Test-Flash. Data read operations
access the internal Flash of the ST10F276E.
5.3.5
Choosing the baud rate for the BSL via UART
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 ST10F276E with a wide range of
baud rates. However, the upper and lower limits must be kept to ensure proper data transfer.
Equation 1
f
CPU
------------------------------------------
B
=
ST10F276E
32 ⋅ (S0BRL + 1)
The ST10F276E 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:
Equation 2
f
T6 – 36
72
9
CPU
S0BRL = -------------------
-- ---------------
,
T6 =
⋅
4 B
Host
For a correct data transfer from the host to the ST10F276E, 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 ST10F276E
baud rate can be calculated using the formula below:
Equation 3
B
– B
Contr
Host
F
=
⋅ 100%
F
≤ 2.5
%
B
---------------------------------------
,
B
B
Contr
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) increases with the host baud rate due to
the smaller baud rate prescaler factors and the implied higher quantization error (see
Figure 10).
Doc ID 12303 Rev 3
57/235
STMICROELECTRONICS [ ST ]
