UPSD3253 datasheet(62/176 Pages) STMICROELECTRONICS

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µPSD325X DEVICES

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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 µPSD325X 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 ap-

plication. The Timer itself can be configured for ei-

ther “timer” or “counter” operation, and in any of its

3 running modes. In the most typical applications,

it is configured for “timer” operation, in the Auto-re-

load 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

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 µPSD325X devices, Timer 2 select-

ed as the baud rate generator by setting TCLK

and/or RCLK (see Figure 22, page 55 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

Table 45. Timer 1-Generated Commonly Used Baud Rates

Baud Rate

fOSC

SMOD

Timer 1

C/T

Mode

Reload Value

Mode 0 Max: 1MHz

12MHz

XXXX

Mode 2 Max: 375K

12MHz

Modes 1, 3: 62.5K

12MHz

FFh

19.2K

11.059MHz

FDh

9.6K

11.059MHz

FDh

4.8K

11.059MHz

FAh

2.4K

11.059MHz

F4h

1.2K

11.059MHz

E8h

137.5

11.059MHz

1Dh

110

6MHz

72h

110

12MHz

FEEBh

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