STC89LE52RC datasheet(114/165 Pages) STC

114 / 165 page

8-Bit UART with Variable Baud Rate (Mode 1)

In mode 1 the STC89xx serial port operates as an 8-bit UART with variable baud rate. A UART, or "universal

asynchronous receiver/transmitter," is a device that receives and transmits serial data with each data character

preceded by a start bit(low) and followed by a stop bit(high). A parity bit is sometimes inserted between the last

data bit and the stop bit. The essential operation of a UART is parallel-to-serial conversion of output data and

serial-to-parallel conversion of input data.

In mode 1, 10 bits are transmitted through TXD or received through RXD. The frame data includes a start bit

(always 0), 8 data bits (LSB first) and a stop bit (always 1). For a receive operation, the stop bit goes into RB8 in

SFR – SCON. The baud rate is determined by the overflow rate of Timer 1 or Timer 2 .

Baud rate in mode 1 = (2

SMOD /32 ) x timer 1 overflow rate

or = (2

SMOD /16 ) x Timer 2 overflow rate

Transmission is initiated by any instruction that uses SBUF as a destination register. The “write to SBUF”

signal also loads a “1” into the 9

th bit position of the transmit shift register and flags the TX Control unit that a

transmission is requested. Transmission actually happens at the next rollover of divided-by-16 counter. Thus the

bit times are synchronized to the divided-by-16 counter, not to the “write to SBUF” signal.

The transmission begins with activation of SEND, which puts the start bit at TXD. One bit time later, DATA is

activated, which enables the output bit of the transmit shift register to TXD. The first shift pulse occurs one bit

time after that.

As data bits shift out to the right, zeroes are clocked in from the left. When the MSB of the data byte is at the

output position of the shift register, then the 1 that was initially loaded into the 9

th position is just to the left of the

MSB, and all positions to the left of that contain zeroes. This condition flags the TX Control unit to do one last

shift and then deactivate SEND and set TI. This occurs at the 10

th divide-by-16 rollover after “write to SBUF.”

Reception is initiated by a 1-to-0 transition detected at RXD. For this purpose, RXD is sampled at a rate of 16

times the established baud rate. When a transition is detected, the divided-by-16 counter is immediately reset,

and 1FFH is written into the input shift register. Resetting the divided-by-16 counter aligns its roll-overs with the

boundaries of the incoming bit times.

The 16 states of the counter divide each bit time into 16ths. At the 7

th, 8th and 9th counter states of each bit time,

the bit detector samples the value of RXD. The value accepted is the value that was seen in at least 2 of the 3

samples. This is done to reject noise. In order to reject false bits, if the value accepted during the first bit time is

not a 0, the receive circuits are reset and the unit continues looking for another 1-to-0 transition. This is to provide

rejection of false start bits. If the start bit is valid, it is shifted into the input shift register, and reception of the rest

of the frame proceeds.

As data bits come in from the right, “1”s shift out to the left. When the start bit arrives at the left most position

in the shift register,(which is a 9-bit register in Mode 1), it flags the RX Control block to do one last shift, load

SBUF and RB8, and set RI. The signal to load SBUF and RB8 and to set RI is generated if, and only if, the

following conditions are met at the time the final shift pulse is generated.

1) RI=0 and

2) Either SM2=0, or SM2=0 and the received stop bit = 1

If either of these two conditions is not met, the received frame is irretrievably lost. If both conditions are met, the

stop bit goes into RB8, the 8 data bits go into SBUF, and RI is activated. At this time, whether or not the above

conditions are met, the unit continues looking for a 1-to-0 transition in RXD.

114

STC89LE52RC 数据表(PDF) 114 Page - STCmicro Technology Co,.Ltd

STC89LE52RC 数据表(HTML) 114 Page - STCmicro Technology Co,.Ltd

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

数据表下载