Philips Semiconductors
P89LPC920/921/922/9221
8-bit microcontrollers with two-clock 80C51 core
Product data Rev. 08 — 15 December 2004 25 of 46
9397 750 14469
© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
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 9
th
data bit, and a stop bit (logical ‘1’). When
data is transmitted, the 9
th
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 9
th
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
1
⁄
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 9
th
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 P89LPC920/921/922/9221 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.
8.18.6 Framing error
Framing error is reported in the status register (SSTAT). In addition, if SMOD0
(PCON.6) is ‘1’, framing errors can be made available in SCON.7 respectively. If
SMOD0 is ‘0’, SCON.7 is SM0. It is recommended that SM0 and SM1 (SCON.7:6)
are set up when SMOD0 is ‘0’.
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
