Manual

High-Speed Microcontroller User’s Guide
Rev: 062210 141 of 176
12.3.2 Mode 2
In this asynchronous mode, baud rates are also generated from the oscillator input. This mode works
identically to the original 8051 family. The baud rate is given by the following formula.
2
SMOD_x
Oscillator Frequency
Mode 1, 3 Baud Rate = 64 12 x (256 - TH1)
The result of this formula generates a baud rate of either 1/32 x oscillator frequency or 1/64 x oscillator
frequency. In the formula, the numerator is expressed as two to the power of SMOD, where SMOD is
either a 0 or 1. When 0, the numerator is a 1 and when SMOD = 1, the numerator is a 2.
SMOD is a bit that effectively doubles the baud rate when set to logic 1. For Serial Port 0, SMOD_0
resides at PCON
.7. This is the original location in the 8051 family. For Serial port 1, SMOD_1 resides in
WDCON
.7. The SMOD bits are set to a logic 0 on reset, which gives the lower speed baud rate.
If the application determines that Mode 0 or 2 must be used, then the oscillator or crystal frequency must
be selected to generate the correct baud rates since each mode offers two selections for a given frequency.
12.3.3 Mode 1 or 3
These asynchronous modes are commonly used for communication with PCs, modems, and other similar
interfaces. The baud rates are programmable using the oscillator input and 16-bit Timer 2 or 8-bit Timer
1. The
respective timer is placed in auto-reload mode. Each time the timer reaches its rollover condition
(FFFFh→0000h—Timer 2 or FFh→00h—Timer 1), a clock is sent to the baud-rate circuit. This clock is
then divided by 16 to generate the exact baud rate. For Serial Port 0, either Timer 1 or 2 can be used to
generate baud rates. Note that there are differences between the timers when used as baud-rate generators.
Serial Port 1 can use Timer 1 as a baud-rate generator. Thus in Mode 1 or 3, the two serial ports can run
at the same frequency if Timer 1 is used for both, but different frequencies if both timers are used.
Also note that the user can determine the speed at which Timer 1 runs (4 clocks or 12 clocks). In most
cases, 12 clocks will be used for baud-rate generation. Timer 2 runs from a two-clock scheme when used
for baud-rate generation. This is compatible with the 80C32.
The baud rates for Mode 1 or 3 are given by these formulas.
Serial Port 0 or 1
2
SMOD_x
Mode 1, 3 Baud Rate = 32 Timer 1 Overflow
Serial Port 0
Timer 2 Overflow
Mode 1, 3 Baud Rate = 16
To use Timer 1 as the baud-rate generator, it is commonly put into the 8-bit auto-reload mode. In this
way, the CPU is not involved in baud-rate generation. Note that the timer interrupt should not be enabled.
In the 8-bit auto-reload mode (Timer 1 Mode 2), the reload value is stored in TH1. Thus the combination
of crystal frequency and TH1 determine the baud rate. The complete formula is as follows.
2
SMOD_x
Oscillator Frequency
Mode 1, 3 Baud Rate = 32 12 x (256 - TH1)
x
x
x