XC2200 Derivatives
System Units (Vol. 1 of 2)
Startup Configuration and Bootstrap Loading
User’s Manual
10-21
V2.1, 2008-08
SCFG/BSL, V1.3
10.6.2.3 Choosing the Baudrate for the BSL
The calculation of the serial baudrate for U0C0 from the length of the first zero byte that
is received, allows the operation of the bootstrap loader of the XC2200 with a wide range
of baudrates. However, the upper and lower limits have to be kept, in order to ensure
proper data transfer.
The XC2200 uses bitfield PDIV to measure the length of the initial zero byte. The
quantization uncertainty of this measurement implies the deviation from the real
baudrate.
For a correct data transfer from the host to the XC2200 the maximum deviation between
the internal initialized baudrate for U0C0 and the real baudrate of the host should be
below 2.5%. The deviation (F
B
, in percent) between host baudrate and XC2200 baudrate
can be calculated via
(10.1)
Note: Function (F
B
) does not consider the tolerances of oscillators and other devices
supporting the serial communication.
This baudrate deviation is a nonlinear function depending on the system clock and the
baudrate of the host. The maxima of the function (F
B
) increase with the host baudrate
due to the smaller baudrate prescaler factors and the implied higher quantization error
(see
).
Figure 10-3 Baudrate Deviation between Host and XC2200
F
B
B
Contr
B
Host
–
B
Contr
-------------------------------------
100%
×
=
F
B
2.5%
≤
MCA02260
B
F
2.5%
Low
B
B
High
Ι
ΙΙ
B
Host