ST10F276Z5 datasheet(57/239 Pages) STMICROELECTRONICS

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ST10F276Z5

Bootstrap loader

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This process may go through several iterations or may directly execute the final application.

In all cases the device 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 device.

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 device with a wide range of baud

rates. However, the upper and lower limits must be kept to ensure proper data transfer.

The device 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 device, 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 device baud rate can be

calculated using 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) increases with the host baud rate due to

the smaller baud rate prescaler factors and the implied higher quantization error (see

Figure 10).

Figure 10.

Baud rate deviation between host and ST10F276Z5

The minimum baud rate (BLow in Figure 10) is determined by the maximum count capacity

of timer T6, when measuring the zero byte, that is, it depends on the CPU clock. Using the

maximum T6 count 216 in the formula the minimum baud rate is calculated. The lowest

fCPU

S0BRL 1

()

⋅

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

BST10F276 =

S0BRL

T6 36

–

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

---

fCPU

BHost

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

⋅

BContr BHost

–

BContr

------------------------------------------- 100

⋅

FB 2.5

≤

BLow

2.5%

BHigh

BHOST

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