C509-L_97 Datasheet, PDF (247/290 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Bootstrap Loader
C509-L
10.3.1 Automatic Synchronization Procedure
In phase II the bootstrap loader starts the serial communication with the host via the serial interface
0 of the C509-L. The interface of the MCU is set to mode 3, which means asynchronous
transmission with the data format 8N2 (eight data bits, no parity, two stop bits). The host has to use
the same serial parameters.
For the baud rate synchronization of the MCU to the fixed baud rate of the host, the bootstrap loader
waits for the testbyte (00H), which has to be sent by the host. By polling the receive port of the serial
interface 0 (P3.0 / RxD) the bootstrap loader measures the receiving time of the test byte by using
timer 0 as shown in the figure 10-7.
Figure 10-7
Measuring the receive time of a zero byte by using Timer 0
The resulting timer value is used to calculate the reload value for the 10-bit baud rate generator of
the serial interface 0 (S0REL). This calculation needs two formulas: the correlation between the
baud rate (Bd) and the reload value (S0REL) depending on the oscillator frequency of the MCU
(fosc)
Bd = ₃---₂-----_×-----₍--₁---₀---₂-^f-^o-₄---^s-–--^c---_S----₀---_R----_E----_L----₎
(1)
and the relation between the baud rate (Bd) and the value of timer 0 (T0) depending on the oscillator
frequency (fosc) and the number of received bits (Nb).
Bd = ^f--^o-_T---^s-₀--^c--_×--^×---₁--^N-₂---^b-
(2)
Equations (1) and (2) and resolving the result to S0REL leads to the formula
S0REL = 1024 – ₃-^T--₂-⁰----_×-^×----_N-¹---^2b-
Semiconductor Group
10-9
1997-10-01