SZF2002 datasheet(36/76 Pages) PHILIPS

36 / 76 page

1998 Aug 26

Philips Semiconductors

Product speciﬁcation

Low voltage 8-bit microcontroller with

6-kbyte embedded RAM

SZF2002

17.3

Baud rates

The baud rate in Mode 0 is fixed and may be calculated as:

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

SMOD bit in Special Function Register PCON and may be

calculated as:

• If SMOD = 0 (value on reset), the baud rate is 1⁄

32fclk

• If SMOD = 1, the baud rate is 1⁄16fclk.

17.3.1

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 the SMOD bit as

follows:

The Timer 1 interrupt should be disabled in this

application. The timer itself can be configured for either

‘timer’ or ‘counter’ operation in any of its 3 running modes.

In typical applications, it is configured for ‘timer’ operation,

in the Auto-reload mode (high nibble of TMOD = 0010B).

In this case the baud rate is given by:

By configuring Timer 1 to run as a 16-bit timer (high nibble

of TMOD = 0001B), and using the Timer 1 interrupt to do

a 16-bit software reload, very low baud rates can be

achieved.

17.3.2

USING TIMER 2 TO GENERATE BAUD RATES

Timer 2 is selected as a Baud Rate Generator by setting

the RCLK0, TCLK0, RCLK1, or TCLK1 bit in T2CON.

The Baud Rate Generator mode is similar to the

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

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

in the registers RCAP2H and RCAP2L, which are preset

by software.

Baud rate

clk

-------

Baud rate

SMOD

-----------------

clk

Baud rate

SMOD

-----------------

Timer 1 overflow rate

Baud rate

SMOD

-----------------

clk

256

TH1

–

()

{}

-----------------------------------------------------

Baud rates in Modes 1 and 3 are determined by Timer 2's

overflow rate as specified below:

Timer 2 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 slightly

different for Timer 2 when it is being used as a Baud Rate

Generator. Normally, as a timer it would increment every

machine cycle at a frequency of 1

⁄

6fclk. However, as a Baud

Rate Generator it increments every state time at a

frequency of fclk. In this case, the baud rate in Modes 1 and

3 is determined as shown by the following equation:

Where (RCAP2H; RCAP2L) is the content of registers

RCAP2H and RCAP2L taken as a 16-bit unsigned integer.

Note that the maximum baud rate depends on clock

frequency and is determined by the following equation:

The Baud Rate Generator mode for Timer 2 is shown in

Fig.16. This figure is only valid if RCLK0 = 1 or TCLK0 = 1

or RCLK1 = 1 or TCLK1 = 1. At roll-over TH2 does not set

the TF2 bit in T2CON and therefore, will not generate an

interrupt. Consequently, the Timer 2 interrupt does not

need to be disabled when in the Baud Rate Generator

mode. If EXEN2 is set, a HIGH-to-LOW transition on T2EX

will set the EXF2 bit, also in T2CON, but will not cause a

reload from (RCAP2H; RCAP2L) to (TH2 and TL2).

Therefore, in this mode T2EX may be used as an

additional external interrupt.

When Timer 2 is operating as a timer (TR2 = 1), in the

Baud Rate Generator mode, registers TH2 and TL2 should

not be accessed (read or write). Under these conditions

the timer increments every state time and therefore the

results of a read or write may not be accurate.

The registers RCAP2H and RCAP2L however, may be

read but not written to. A write might overlap a reload and

cause write and/or reload errors. If a write operation is

required, Timer 2 or RCAP2H/RCAP2L should first be

turned off by clearing the TR2 bit.

Baud rate

Timer 2 overflow rate

--------------------------------------------------------

Baud rate

clk

65536

RCAP2H; RCAP2L

()

–

{}

------------------------------------------------------------------------------------------------------

Maximum baud rate

clk

----------------

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