C515A_9708 Datasheet, PDF (98/182 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C515A
The baud rate generator consists of a free running upward counting 10-bit timer. On overflow of this
timer (next count step after counter value 3FFH) there is an automatic 10-bit reload from the
registers SRELL and SRELH. The lower 8 bits of the timer are reloaded from SRELL, while the
upper two bits are reloaded from bit 0 and 1 of register SRELH. The baud rate timer is reloaded by
writing to SRELL.
Special Function Register SRELH (Address BAH)
Special Function Register SRELL (Address AAH)
Reset Value : XXXXXX11B
Reset Value : D9H
Bit No. MSB
LSB
7
6
5
4
3
2
1
0
BAH
–
–
–
–
–
–
MSB
.9
.8
SRELH
AAH .7
.6
.5
.4
.3
.2
.1
LSB
.0
SRELL
The shaded bits are don’t care for reload operation.
Bit
SRELH.0-1
SRELL.0-7
Function
Baudrate generator reload high value
Upper two bits of the baudrate timer reload value.
Baudrate generator reload low value
Lower 8 bits of the baudrate timer reload value.
After reset SRELH and SRELL have a reload value of 3D9H. With this reload value the baud rate
generator has an overflow rate of input clock / 39 (compatible to C515). With 12 MHz oscillator
frequency, the commonly used baud rates 4800 baud (SMOD = 0) and 9600 baud (SMOD = 1) are
available (with 0.16 % deviation).
With the baud rate generator as clock source for the serial port in mode 1 and 3, the baud rate of
can be determined as follows:
Mode 1, 3 baud rate =
2^SMOD x oscillator frequency
64 x (baud rate generator overflow rate)
Baud rate generator overflow rate = 2¹⁰ – SREL
with SREL = SRELH.1 – 0, SRELL.7 – 0
Semiconductor Group
6-47
1997-08-01