Analog Devices ADSP-BF59x Blackfin, Hardware Reference Manual, Page: 370 / 772

Description of Operation
11-14 ADSP-BF59x Blackfin Processor Hardware Reference
clock oscillator that drifts over time, the Blackfin can re-adjust its UART
bit rate dynamically.
Often, autobaud detection is used for initial bit rate negotiations. In this
case, the Blackfin processor is most likely a slave device waiting for the
host to send a predefined autobaud character (see below). This is the sce-
nario used for UART booting. In this scenario, the UART clock enable bit
UCEN
should not be enabled while autobaud detection is performed. This
prevents the UART from starting reception with incorrect bit rate match-
ing. Alternatively, the UART can be disconnected from the
RX
pin by
setting the
LOOP_ENA
bit.
A software routine can detect the pulse widths of serial stream bit cells.
Because the sample base of the timers is synchronous with the UART
operation—all derived from
SCLK
—the pulse widths can be used to calcu-
late the baud rate divider for the UART.
divisor =
TIMER_WIDTH
/(16 × number of captured UART bits)
In order to increase the number of timer counts and therefore the resolu-
tion of the captured signal, it is recommended not to measure just the
pulse width of a single bit, but to enlarge the pulse of interest over more
bits. Traditionally, a NULL character (ASCII 0x00) was used in autobaud
detection, as shown in
Figure 11-5 .
Because the example frame in Figure 11-5 encloses 8 data bits and 1 start
bit, apply the formula:
divisor =
TIMER_WIDTH
/(16 × 9)
Figure 11-5. Autobaud Detection Character 0x00
FRAME WIDTH
S 1 2 3 4 5 6 7 STOP 0