DS80C320-MCG Datasheet, PDF (143/175 Pages) Dallas Semiconductor – High-Speed Microcontroller User Guide

T2CON.4
High-Speed Microcontroller User’s Guide
TCLK - Transmit Clock Flag. This bit determines whether Timer 1
or 2 is used for Serial Port 0 timing of Transmit data in Serial
Modes 1 or 3. TCLK = 1 causes Timer 2 overflow to be used as the
transmit clock. TCLK = 0 causes Timer 1 overflow to be used as
the transmit clock.
BAUD RATES
Each mode has a baud rate generator associated with it. This generator is generally the same for each
UART. Several of the baud rate generation techniques have options and these options are independent
for the two UARTs. The baud rate descriptions given below are separated by mode.
Mode 0
Baud rates for this mode are driven directly from the crystal speed divided by either 12 or 4. Mode 0 is
synchronous so that the shift clock output frequency will be the baud rate. The formula is simply as
follows:
Oscillator Frequency
Mode 0 baud rate =
12
or
Oscillator Frequency
Mode 0 baud rate =
4
The default case is divide by 12. The user can select the using the SM2 bit in the associated SCON
register. For Serial Port 0, the SM2_0 bit is SCON0.5. For Serial Port 1, the SM2_0 bit is SCON1.5.
When SM2 is set to a logic 0, the baud rate is generated using a divide by 12 of the oscillator input.
When SM2 is set to a logic 1, the baud rate is generated using divide by 4. Note that this use of SM2
differs from a standard 80C32. In that device, SM2 had no valid use when the UART was in Mode 0.
Since it was generally set to a 0, for the divide by 12, there is no compatibility problem.
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 * (256 - TH1)
The result of this formula generates a baud rate of either1/32 * oscillator frequency or 1/64 * 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 a 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.
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