UPSD3312D datasheet(102/272 Pages) STMICROELECTRONICS

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Doc ID 9685 Rev 7

20.6.3

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 26 on page 105

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 Section Table 31.: PCON: Power

Control register (SFR 87h, reset value 00h) on page 72) have identical functions as RCLK

and TCLK but they apply to UART1 instead. For simplicity in the following discussions about

baud rate generation, no suffix will be used when referring to SFR registers and bits related

to UART0 or UART1, since each UART interface 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 determined 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

applications, 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 unsigned integer.

A roll-over in TH2 does not set TF2, and will not generate an interrupt. Therefore, the Timer

Interrupt 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,

firmware 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 61 on page 103 shows commonly used baud rates and how they can be obtained

from Timer 2, with T2CON = 34h.

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