Asynchronous Serial Ports (UARTs)
Am186™CC/CH/CU Microcontrollers User’s Manual
13-17
Figure 13-7
Worst Case % Error Per Bit vs. Baud Divisor Without Autobaud Enhancement
Figure 13-8
Detectable Baud Ranges for Various Frequencies
The microcontroller also offers an enhancement to autobaud detection. Start bit width
distortion can result in calculation errors. In instances where there is some system
knowledge, baud divisor and threshold values can be programmed to allow for a 25–30%
distortion in the width of the start bit.This method greatly increases the probability that the
correct valid baud rate divisor is selected for higher baud rates, which are at highest risk.
The microcontroller method makes use of the fact that, in most cases, valid baud divisor
values used in a particular application are separated by several integers. Four High-Speed
Serial Port Autobaud registers are provided for this enhancement: HSPAB0, HSPAB1,
HSPAB2 and HSPAB3. Each register contains a divisor value and a threshold value. The
HSPAB3 register must contain the largest programmed divisor value and threshold value,
then HSPAB2, etc. When using fewer than four valid divisor values, software must clear the
unused HSPABx registers or leave them at their default values (00h).
When the registers have been programmed, the High-Speed UART compares the autobaud
calculated baud rate divisor to the threshold values and selects one of the programmed
valid baud rate divisors to use in subsequent data transfers. A calculated value less than
5
10
15
20
25
30
35
40
1
2
3
4
5
6
7
8
9
10
11
166
167
Worst case % error per bit = 1 – BAUD DIV
BAUD DIV + .5
W
o
rs
t Ca
s
e
%
Error Pe
r
B
it
Baud Divisor
33%
.3%
.29%
4.3%
Error Range
Worst Case % Error
50
45
20
25
30
35
40
Baud
Rates
(Kba
ud)
Frequency (MHz)
2
4
6
8
10
12
14
16
18
Questionable
(Baud Divisors < 166)
Autobaud Detection
Baud Divisor = 166
Guaranteed
(Baud Divisors > 166)
Autobaud Detection
Summary of Contents for Am186 CC
Page 1: ...Am186 CC CH CU Microcontrollers User s Manual Order 21914B...
Page 4: ...iv Am186 CC CH CU Microcontrollers User s Manual...
Page 18: ...Table of Contents xviii Am186 CC CH CU Microcontrollers User s Manual...
Page 24: ...Introduction xxiv Am186 CC CH CU Microcontrollers User s Manual...
Page 40: ...Architectural Overview 1 16 Am186 CC CH CU Microcontrollers User s Manual...
Page 86: ...System Overview 3 36 Am186 CC CH CU Microcontrollers User s Manual...
Page 92: ...Emulator Support 4 6 Am186 CC CH CU Microcontrollers User s Manual...
Page 112: ...DRAM Controller 6 8 Am186 CC CH CU Microcontrollers User s Manual...
Page 134: ...Interrupts 7 22 Am186 CC CH CU Microcontrollers User s Manual...
Page 186: ...Programmable I O Signals 9 8 Am186 CC CH CU Microcontrollers User s Manual...
Page 200: ...Watchdog Timer 11 6 Am186 CC CH CU Microcontrollers User s Manual...
Page 232: ...Asynchronous Serial Ports UARTs 13 24 Am186 CC CH CU Microcontrollers User s Manual...
Page 242: ...Synchronous Serial Port SSI 14 10 Am186 CC CH CU Microcontrollers User s Manual...
Page 264: ...High Level Data Link Control HDLC 15 22 Am186 CC CH CU Microcontrollers User s Manual...
Page 332: ...Universal Serial Bus USB 18 34 Am186 CC CH CU Microcontrollers User s Manual...
Page 348: ...Register Summary A 16 Am186 CC CH CU Microcontrollers User s Manual...
Page 376: ...Index Index 18 Am186 CC CH CU Microcontrollers User s Manual...