UM10147_2
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 02 — 28 April 2008
63 of 134
NXP Semiconductors
UM10147
P89LPC952/954 User manual
Reception is initiated by clearing RI_n (SnCON.0). Synchronous serial transfer occurs
and RI_n will be set again at the end of the transfer. When RI_n is cleared, the reception
of the next character will begin. Refer to
10.11 More about UART Mode 1
Reception is initiated by detecting a 1-to-0 transition on RXDn. RXDn is sampled at a rate
16 times the programmed baud rate. When a transition is detected, the divide-by-16
counter is immediately reset. Each bit time is thus divided into 16 counter states. At the
7th, 8th, and 9th counter states, the bit detector samples the value of RXDn. The value
accepted is the value that was seen in at least 2 of the 3 samples. This is done for noise
rejection. If the value accepted during the first bit time is not 0, the receive circuits are
reset and the receiver goes back to looking for another 1-to-0 transition. This provides
rejection of false start bits. If the start bit proves valid, it is shifted into the input shift
register, and reception of the rest of the frame will proceed.
The signal to load SnBUF and RB8_n, and to set RI_n, will be generated if, and only if, the
following conditions are met at the time the final shift pulse is generated: RI_n = 0 and
either SM2_n = 0 or the received stop bit = 1. If either of these two conditions is not met,
the received frame is lost. If both conditions are met, the stop bit goes into RB8_n, the 8
data bits go into S0BUF, and RI_n is activated.
Fig 22. Serial Port Mode 0 (double buffering must be disabled).
002aaa925
transmit
RXD (data out)
RXD
(data in)
D0
D1
D5
D2
D6
D3
D4
D7
TXD (shift clock)
shift
S1 ... S16
S1 ... S16 S1 ... S16 S1 ... S16
S1 ... S16
S1 ... S16 S1 ... S16 S1 ... S16
S1 ... S16
S1 ... S16
S1 ... S16
S1 ... S16
S1 ... S16
write to
SBUF
TI
receive
D0
D1
D5
D2
D6
D3
D4
D7
TXD (shift clock)
shift
WRITE to SCON
(clear RI)
RI