T83C5121XXX-ICSIL datasheet(86/115 Pages) ATMEL

86 / 115 page

A/T8xC5121

4164G–SCR–07/06

UART Timings

The following description will be included in L version:

Mode Selection

SM0 and SM1 bits in SCON register (see Table 67) are used to select a mode among

the single synchronous and the three asynchronous modes according to Table 64.

Table 64. Serial I/O Port Mode Selection

Baud Rate Generator

Depending on the mode and the source selection, the baud rate can be generated from

either the Timer 1 or the Internal Baud Rate Generator. The Timer 1 can be used in

Modes 1 and 3 while the Internal Baud Rate Generator can be used in Modes 0, 1 and

The addition of the Internal Baud Rate Generator allows freeing of the Timer 1 for other

purposes in the application. It is highly recommended to use the Internal Baud Rate

Generator as it allows higher and more accurate baud rates than with Timer 1.

Baud rate formulas depend on the modes selected and are given in the following mode

sections.

Timer 1

When using the Timer 1, the Baud Rate is derived from the overflow of the timer. As

shown in Figure 36 the Timer 1 is used in its 8-bit auto-reload mode (detailed in

Section “Timer 0/Counter Operations”, page 73). SMOD1 bit in PCON register allows

doubling of the generated baud rate.

Figure 36. Timer 1 Baud Rate Generator Block Diagram

Internal Baud Rate Generator

When using the Internal Baud Rate Generator, the Baud Rate is derived from the over-

flow of the timer. As shown in Figure 37, the Internal Baud Rate Generator is an 8-bit

auto-reload timer feed by the peripheral clock or by the peripheral clock divided by 6

depending on the SPD bit in BDRCON register (see Table 68). The Internal Baud Rate

Generator is enabled by setting BBR bit in BDRCON register. SMOD1 bit in PCON reg-

ister allows doubling of the generated baud rate.

SM0

SM1

Mode

Description

Baud Rate

Synchronous Shift Register

Fixed / Variable

8-bit UART

Variable

9-bit UART

Fixed

9-bit UART

Variable

TR1

TCON.6

GATE1

TMOD.7

Overflow

C/T1#

TMOD.6

TL1

(8 bits)

TH1

(8 bits)

INT1

PER

CLOCK

÷ 6

SMOD1

PCON.7

÷ 2

CLOCK

To Serial Port

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