UART/Autobaud Functional Description
12-49
UART Devices
The following settings are allowed in autobaud mode:
-
Speed:
115.2K baud, 57.6K baud, 38.4K baud, 28.8K baud, 19.2K baud, 14.4K
baud, 9.6K baud, 4.8K baud, 2.4K baud, or 1.2K baud.
-
Length: 7 or 8 bits
-
Parity: Odd, even, or space
Combination 7-bit space parity is forbidden.
The method to identify the speed is:
1) Detect the transition 1->0 on the received data. This happens as soon as
a stop to start bit transition occurs. The transition is valid after a majority
vote on three sampling periods.
2) Sample the start bit duration with 115 200 *16 Hz clock frequency as long
as there is no rising edge. A transition 0->1 is considered as valid after a
majority vote on three sampling periods.
3) Compare the sampled value with a table. If the sampled value is outside
a valid range, an error is reported (no speed identified), and the hardware
goes back to the first state (1).
4) Otherwise, the first data bit in the received register (for serial to parallel
conversion) is stored and goes to frame format identification.
5) The next received bits are sampled according to the programmed baud
rate. After reception of seven bits, the speed identification must be
restarted, because you may receive several a or A characters before a
valid t or T character.
Autobaud mode is selected when MDR1[2:0] = 010. In UART autobaud mode,
DLL, DLH, and LCR[5:0] settings are not used. Instead, UASR is updated with
the configuration detected by the autobauding logic (see Figure 12–13).