UPSD3233 datasheet(57/170 Pages) STMICROELECTRONICS

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uPSD3234A, uPSD3234BV, uPSD3233B, uPSD3233BV

Table 44. Description of the SCON Bits

Baud Rates. The baud rate in Mode 0 is fixed:

Mode 0 Baud Rate = fOSC / 12

The baud rate in Mode 2 depends on the value of

Bit SMOD = 0 (which is the value on reset), the

baud rate is 1/64 the oscillator frequency. If SMOD

= 1, the baud rate is 1/32 the oscillator frequency.

Mode 2 Baud Rate = (2SMOD / 64) x fOSC

In the uPSD323X Devices, the baud rates in

Modes 1 and 3 are determined by the Timer 1

overflow rate.

Using Timer 1 to Generate 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 (see:

Mode 1,3 Baud Rate =

(2SMOD / 32) x (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 3 running modes. In the most typical applica-

tions, it is configured for “timer” operation, in the

Auto-reload Mode (high nibble of TMOD = 0010B).

In that case the baud rate is given by the formula:

Mode 1,3 Baud Rate =

(2SMOD / 32) x (fOSC / (12 x [256 – (TH1)]))

One can achieve very low baud rates with Timer 1

by leaving the Timer 1 Interrupt enabled, and con-

figuring the Timer to run as a 16-bit timer (high nib-

ble of TMOD = 0001B), and using the Timer 1

Interrupt to do a 16-bit software reload. Figure

22., page 51 lists various commonly used baud

rates and how they can be obtained from Timer 1.

Using Timer/Counter 2 to Generate Baud

Rates. In the uPSD323X Devices, Timer 2 select-

ed as the baud rate generator by setting TCLK

and/or RCLK (see Figure 22., page 51 Timer/

Counter 2 Control Register (T2CON)).

Note: The baud rate for transmit and receive can

be simultaneously different. Setting RCLK and/or

TCLK puts Timer into its Baud Rate Generator

Mode.

The RCLK and TCLK Bits in the T2CON register

configure UART 1. The RCLK1 and TCLK1 Bits in

the PCON register configure UART 2.

The Baud Rate Generator Mode is similar to the

Auto-reload Mode, in that a roll over in TH2 causes

the Timer 2 registers to be reloaded with the 16-bit

value in registers RCAP2H and RCAP2L, which

are preset by software.

Now, the baud rates in Modes 1 and 3 are deter-

mined at Timer 2’s overflow rate as follows:

Mode 1,3 Baud Rate = Timer 2 Overflow Rate / 16

Bit

Symbol

Function

SM0

(SM1,SM0)=(0,0): Shift Register. Baud rate = fOSC/12

(SM1,SM0)=(1,0): 8-bit UART. Baud rate = variable

(SM1,SM0)=(0,1): 8-bit UART. Baud rate = fOSC/64 or fOSC/32

(SM1,SM0)=(1,1): 8-bit UART. Baud rate = variable

6SM1

5SM2

Enables the multiprocessor communication features in Mode 2 and 3. In Mode 2 or 3, if

SM2 is set to '1,' RI will not be activated if its received 8th data bit (RB8) is '0.' In Mode 1,

if SM2=1, RI will not be activated if a valid Stop Bit was not received. In Mode 0, SM2

should be '0'

4REN

Enables serial reception. Set by software to enable reception. Clear by software to

disable reception

3TB8

The 8th data bit that will be transmitted in Modes 2 and 3. Set or clear by software as

desired

2RB8

In Modes 2 and 3, this bit contains the 8th data bit that was received. In Mode 1, if

SM2=0, RB8 is the Snap Bit that was received. In Mode 0, RB8 is not used

1TI

Transmit Interrupt Flag. Set by hardware at the end of the 8th bit time in Mode 0, or at

the beginning of the Stop Bit in the other modes, in any serial transmission. Must be

cleared by software

0RI

Receive Interrupt Flag. Set by hardware at the end of the 8th bit time in Mode 0, or

halfway through the Stop Bit in the other modes, in any serial reception (except for

SM2). Must be cleared by software

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