C509-L_97 Datasheet, PDF (248/290 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Bootstrap Loader
C509-L
which is independent from the oscillator frequency of the MCU (fosc). The value of Nb is nine,
because one start bit plus eight data bits are measured. The resulting formula then is
S0REL = 1024 – -^T-₃--⁰-₂----^×-_×---¹-₉--²--
This equation contains the constant factor
3^---2^--1---×^2-----9^{- = 0.0417}
So the formula can be written as
S0REL = 1024 – 0.0147 × T0
To avoid complicated float point arithmetic the factor 0.0417 is scaled by multiplying it with 4096
(result is 171) and then performing an integer multiplication with T0. In the next step the product is
rescaled by a integer division with 4096, which can be simply achieved by a twelve bit right-shift
operation. The final formula for calculating the reload value S0REL including scaling and rescaling
is therefore:
S0REL = 1024 – -¹--⁷--₄-¹--₀---^×₉---₆--^T---⁰--
(3)
Additionally, the result of the division is rounded by a simple bit comparison of the last right shifted
bit. After setting S0REL to the calculated value and activating the baud rate generator of the serial
interface 0, the bootstrap loader sends an acknowledge byte (55H) to the host. If this byte is
received correctly, it will be assured, that both serial interfaces are working with the same baud rate.
The flowchart in figure 10-8 shows the calculation of the reload value S0REL for the baud rate
generator of the serial interface 0.
Semiconductor Group
10-10
1997-10-01