CME136686LX Hardware Manual - RTD Embedded Technologies ...

Non-Standard Serial Port Modes
It is possible to change the input clock rate for the UARTs of the cpuModule by selecting the Serial Port Baud
Rates option in the Serial Port Configuration menu of the BIOS Setup. Changing the option from Normal to
Non-Standard will allow the serial port to operate at higher speeds.
This transforms bits [7:5] of the Divisor Latch High Byte of the UART into selections for alternate clock rates. The
following table describes the bit operations and the resulting divide-by-one baud rate:
Table 40 Divisor Latch High and Low Bytes
Divisor Latch High Byte
Bit 7 Bit 6 Bit 5 Bits [4:0]
Divisor
Latch Low
Byte
Baud Rate
% Error
0 0 0 0x00 0x01 115,200 0.16
1 0 0 0x00 0x01 460,800 0.16
1 1 0 0x00 0x01 921,600 0.16
0 0 1 0x00 0x01 1,500,000 0.16
To achieve non-standard baud rates, divide the baud rate you require by one of the non-standard divisors
(460,800, 921,600, and 1,500,000). If the result is a whole number, substitute that value for the Divisor Latch Low
Byte. For example, to achieve a baud rate of 750,000, select the Divisor Latch High Byte for 1,500,000 and set the
Divisor Latch Low Byte to 2.
Note The signaling mode of the output will limit the highest baud rate achievable. For RS-232 mode
the maximum suggested baud rate is 230,400. For 422/485 modes the maximum is 1,500,000.
Note When using the non-standard high speed serial port modes, it is highly recommended to use
hardware flow control, whenever possible.
66 CME136686LX cpuModule BDM-610000065 Rev C