UPSD3313D-40T6 datasheet(77/231 Pages) STMICROELECTRONICS

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uPSD33xx

Baud Rate Generator Mode. The RCLK and/or

TCLK Bits in the SFR T2CON allow the transmit

and receive baud rates on serial port UART0 to be

derived from either Timer 1 or Timer 2. Figure

27., page 80 illustrates Baud Rate Generator

Mode.

When TCLK = 0, Timer 1 is used as UART0’s

transmit baud generator. When TCLK = 1, Timer 2

will be the transmit baud generator. RCLK has the

same effect for UART0’s receive baud rate. With

these two bits, UART0 can have different receive

and transmit baud rates - one generated by Timer

1, the other by Timer 2.

Note: Bits RCLK1 and TCLK1 in the SFR named

PCON (see PCON: Power Control Register (SFR

87h, reset value 00h), page 50) have identical

functions as RCLK and TCLK but they apply to

UART1 instead. For simplicity in the following dis-

cussions about baud rate generation, no suffix will

be used when referring to SFR registers and bits

related to UART0 or UART1, since each UART in-

terface has identical operation. Example, TCLK or

TCLK1 will be referred to as just TCLK.

The Baud Rate Generator Mode is similar to the

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

the Timer 2 registers, TH2 and TL2, to be reloaded

with the 16-bit value in Registers RCAP2H and

RCAP2L, which are preset with firmware.

The baud rates in UART Modes 1 and 3 are deter-

mined by Timer 2’s overflow rate as follows:

UART Mode 1,3 Baud Rate =

Timer 2 Overflow Rate / 16

The timer can be configured for either “timer” or

“counter” operation. In the most typical applica-

tions, it is configured for “timer” operation (C/T2 =

0). “Timer” operation is a little different for Timer 2

when it's being used as a baud rate generator. In

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

UART Mode 1,3 Baud Rate =

fOSC/(32 x [65536 – [RCAP2H, RCAP2L]))

where [RCAP2H, RCAP2L] is the content of the

SFRs RCAP2H and RCAP2L taken as a 16-bit un-

signed integer.

A roll-over in TH2 does not set TF2, and will not

generate an interrupt. Therefore, the Timer Inter-

rupt does not have to be disabled when Timer 2 is

in the Baud Rate Generator Mode.

If EXEN2 is set, a 1-to-0 transition on pin T2X will

set the Timer 2 interrupt flag EXF2, but will not

cause a reload from RCAP2H and RCAP2L to

TH2 and TL2. Thus when Timer 2 is in use as a

baud rate generator, the pin T2X can be used as

an extra external interrupt, if desired.

When Timer 2 is running (TR2 = 1) in a “timer”

function in the Baud Rate Generator Mode, firm-

ware should not read or write TH2 or TL2. Under

these conditions the results of a read or write may

not be accurate. However, SFRs RCAP2H and

RCAP2L may be read, but should not be written,

because a write might overlap a reload and cause

write and/or reload errors. Timer 2 should be

turned off (clear TR2) before accessing Timer 2 or

Registers RCAP2H and RCAP2L, in this case.

Table 43., page 78 shows commonly used baud

rates and how they can be obtained from Timer 2,

with T2CON = 34h.

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