C509-L_97 Datasheet, PDF (250/290 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Bootstrap Loader
C509-L
10.3.2 Baud Rates for Correct Synchronization
The automatic baud rate synchronization will work correctly only in a specific range of baud rates,
which depends on the oscillator frequency fosc of the C509-L and the resolution of the timer 0 (T0).
The minimum baud rate (Bdmin) results in the possible underflow of SFR S0REL when the value of
T0 gets greater than 24556. In this case the value of SFR S0REL, corresponding formula (3), gets
below zero, which leads to a underflow of the S0REL register and therefore to a incorrect baud rate
of the baud rate generator. The formula for calculating this underflow margin is derived from formula
(1) and is reduced to
Bdmin = ₃--^f-₂-^o--₇--^s-₆--^c-₈--
The theoretical maximum baud rate (Bdhigh) can be attained if S0REL is set to its maximum value
of 1023. In this case formula (1) reduces to
Bdhigh = ^f--^o-₃---^s2---^c-
The real maximum baud rate (Bdmax) is smaller, because of the decreasing resolution of S0REL and
T0 at higher baud rates, This causes an increasing deviation between the host baud rate and the
MCU baud rate. To perform a correct transfer between the MCU and the host without transmission
errors, the deviation Fb of the host baud rate to the MCU baud rate may not exceed 2.5%.
Fb = 0.025 ≤ ^B----^d----^h--^o---^s--^t---–----^B----^d----^M---^C---^U--
Bdhost
Between Bdmin and Bdmax every host baud rate can be synchronized successfully by the bootstrap
loader. Above Bdmax only discreet baud rates with a deviation of less than 2.5 % may be used.
Table 10-2 shows the guaranteed range of baud rates Bdmin to Bdmax and typical functional host
baud rates above Bdmax for some MCU clock rates fosc.
Table 10-2
Typical Baudrate Selections
MCU clock rate
fosc
8 MHz
12 MHz
16 MHz
Minimum baud rate
Bdmin
250 baud
370 baud
490 baud
Maximum baud rate
Bdmax (F_b 2,5%)
6580 baud
9380 baud
12830 baud
Functional higher baud
rates
9600, 19200 baud
9600, 19200 baud
19200, 38400 baud
Semiconductor Group
10-12
1997-10-01