AD822 datasheet(67/88 Pages) AD

67 / 88 page

ADuC841/ADuC842/ADuC843

Rev. 0 | Page 67 of 88

Mode 3: 9-Bit UART with Variable Baud Rate

Mode 3 is selected by setting both SM0 and SM1. In this mode,

the 8051 UART serial port operates in 9-bit mode with a vari-

able baud rate determined by either Timer 1 or Timer 2. The

operation of the 9-bit UART is the same as for Mode 2, but the

baud rate can be varied as for Mode 1.

In all four modes, transmission is initiated by any instruction

that uses SBUF as a destination register. Reception is initiated in

Mode 0 by the condition RI = 0 and REN = 1. Reception is

initiated in the other modes by the incoming start bit if REN = 1.

UART Serial Port Baud Rate Generation

Mode 0 Baud Rate Generation

The baud rate in Mode 0 is fixed.

Mode 0 Baud Rate = (Core Clock Frequency/12)

Mode 2 Baud Rate Generation

The baud rate in Mode 2 depends on the value of the SMOD bit

in the PCON SFR. If SMOD = 0, the baud rate is 1/32 of the

core clock. If SMOD = 1, the baud rate is 1/16 of the core clock:

Mode 2 Baud Rate = (2SMOD/32 × [Core Clock Frequency])

Modes 1 and 3 Baud Rate Generation

The baud rates in Modes 1 and 3 are determined by the over-

flow rate in Timer 1 or Timer 2, or in both (one for transmit

and the other for receive).

Timer 1 Generated Baud Rates

When Timer 1 is used as the baud rate generator, the baud rates

in Modes 1 and 3 are determined by the Timer 1 overflow rate

and the value of SMOD as follows:

Modes 1 and 3 Baud Rate = (2SMOD/32 × (Timer 1 Overflow Rate)

The Timer 1 interrupt should be disabled in this application.

The timer itself can be configured for either timer or counter

operation, and in any of its three running modes. In the most

typical application, it is configured for timer operation in the

autoreload mode (high nibble of TMOD = 0010 binary). In that

case, the baud rate is given by the formula

Modes 1 and 3 Baud Rate =

(2SMOD/32) × (Core Clock/ [256 − TH1])

Timer 2 Generated Baud Rates

Baud rates can also be generated using Timer 2. Using Timer 2

is similar to using Timer 1 in that the timer must overflow 16

times before a bit is transmitted/received. Because Timer 2 has a

16-bit autoreload mode, a wider range of baud rates is possible

using Timer 2.

Modes 1 and 2 Baud Rate = (1/16) × (Timer 2 Overflow Rate)

Therefore, when Timer 2 is used to generate baud rates, the

timer increments every two clock cycles rather than every core

machine cycle as before. Thus, it increments six times faster

than Timer 1, and therefore baud rates six times faster are possi-

ble. Because Timer 2 has 16-bit autoreload capability, very low

baud rates are still possible.

Timer 2 is selected as the baud rate generator by setting the

TCLK and/or RCLK in T2CON. The baud rates for transmit

and receive can be simultaneously different. Setting RCLK and/

or TCLK puts Timer 2 into its baud rate generator mode as

shown in Figure 73.

In this case, the baud rate is given by the formula

Modes 1 and 3 Baud Rate =

(Core Clock)/(16 × [65536 − (RCAP 2H, RCAP 2L)])

CORE

CLK*

TR2

CONTROL

TL2

(8 BITS)

TH2

(8 BITS)

RELOAD

EXEN2

CONTROL

T2EX

TRANSITION

DETECTOR

EXF 2

TIMER 2

INTERRUPT

NOTE: AVAILABILITY OF ADDITIONAL

EXTERNAL INTERRUPT

RCAP2L

RCAP2H

TIMER 2

OVERFLOW

RCLK

TCLK

CLOCK

SMOD

TIMER 1

OVERFLOW

C/ T2 = 0

C/ T2 = 1

*CORE CLK IS DEFINED BY THE CD BITS IN PLLCON

Figure 73. Timer 2, UART Baud Rates

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