P89LPC921 datasheet(24/45 Pages) PHILIPS

24 / 45 page

Philips Semiconductors

P89LPC920/921/922

8-bit microcontrollers with two-clock 80C51 core

Product data

Rev. 06 — 21 November 2003

24 of 45

9397 750 12285

8.18.1

Mode 0

Serial data enters and exits through RxD. TxD outputs the shift clock. 8 bits are

transmitted or received, LSB ﬁrst. The baud rate is ﬁxed at 1

⁄

16 of the CPU clock

frequency.

8.18.2

Mode 1

10 bits are transmitted (through TxD) or received (through RxD): a start bit

(logical ‘0’), 8 data bits (LSB ﬁrst), and a stop bit (logical ‘1’). When data is received,

the stop bit is stored in RB8 in Special Function Register SCON. The baud rate is

variable and is determined by the Timer 1 overﬂow rate or the Baud Rate Generator

(described in Section 8.18.5 “Baud rate generator and selection”).

8.18.3

Mode 2

11 bits are transmitted (through TxD) or received (through RxD): start bit (logical ‘0’),

8 data bits (LSB ﬁrst), a programmable 9th data bit, and a stop bit (logical ‘1’). When

data is transmitted, the 9th data bit (TB8 in SCON) can be assigned the value of ‘0’ or

‘1’. Or, for example, the parity bit (P, in the PSW) could be moved into TB8. When

data is received, the 9th data bit goes into RB8 in Special Function Register SCON,

while the stop bit is not saved. The baud rate is programmable to either 1

⁄

16 or

⁄

32 of

the CPU clock frequency, as determined by the SMOD1 bit in PCON.

8.18.4

Mode 3

11 bits are transmitted (through TxD) or received (through RxD): a start bit

(logical ‘0’), 8 data bits (LSB ﬁrst), a programmable 9th data bit, and a stop bit

(logical ‘1’). In fact, Mode 3 is the same as Mode 2 in all respects except baud rate.

The baud rate in Mode 3 is variable and is determined by the Timer 1 overﬂow rate or

the Baud Rate Generator (described in Section 8.18.5 “Baud rate generator and

selection”).

8.18.5

Baud rate generator and selection

The P89LPC920/921/922 enhanced UART has an independent Baud Rate

Generator. The baud rate is determined by a baud-rate preprogrammed into the

BRGR1 and BRGR0 SFRs which together form a 16-bit baud rate divisor value that

works in a similar manner as Timer 1 but is much more accurate. If the baud rate

generator is used, Timer 1 can be used for other timing functions.

The UART can use either Timer 1 or the baud rate generator output (see Figure 7).

Note that Timer T1 is further divided by 2 if the SMOD1 bit (PCON.7) is set. The

independent Baud Rate Generator uses OSCCLK.

Fig 7.

Baud rate sources for UART (Modes 1, 3).

Baud Rate Modes 1 and 3

SBRGS = 1

SBRGS = 0

SMOD1 = 0

SMOD1 = 1

¸2

Timer 1 Overflow

(PCLK-based)

Baud Rate Generator

(CCLK-based)

002aaa419

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