Z51F0410 Datasheet, PDF (106/184 Pages) Zilog, Inc. – Z8051 Series 8-Bit Microcontrollers

Z51F0410
Product Specification
4.6.9.4 Disabling Receiver
In contrast to Transmitter, disabling the Receiver by clearing RXE bit makes the Receiver inactive
immediately. When the Receiver is disabled the Receiver flushes the receive buffer and the remaining
data in the buffer is all reset. The RXD pin is not overridden the function of USART, so RXD pin
becomes normal GPIO or primary function pin.
4.6.9.5 Asynchronous Data Reception
To receive asynchronous data frame, the USART includes a clock and data recovery unit. The Clock
Recovery logic is used for synchronizing the internally generated baud-rate clock to the incoming
asynchronous serial frame on the RXD pin.
The Data recovery logic samples and low pass filters the incoming bits, and this removes the noise
of RXD pin.
The next figure illustrates the sampling process of the start bit of an incoming frame. The sampling
rate is 16 times the baud-rate for normal mode, and 8 times the baud rate for Double Speed mode
(U2X=1). The horizontal arrows show the synchronization variation due to the asynchronous sampling
process. Note that larger time variation is shown when using the Double Speed mode.
RxD
IDLE
START
BIT0
Sample
(U2X = 0)
Sample
(U2X = 1)
0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2 3
0
1
2
3
4
5
6
7
8
1
2
When the Receiver is enabled (RX^FE^ig=^u1^re), ⁴th^.2e² c^Sl^to^ac^rk^{t B}re^itc^So^av^mer^py^lilⁿo^ggic tries to find a high to low transition
on the RXD line, the start bit condition. After detecting high to low transition on RXD line, the clock
recovery logic uses samples 8,9, and 10 for Normal mode, and samples 4, 5, and 6 for Double Speed
mode to decide if a valid start bit is received. If more than 2 samples have logical low level, it is
considered that a valid start bit is detected and the internally generated clock is synchronized to the
incoming data frame. And the data recovery can begin. The synchronization process is repeated for
each start bit.
As described above, when the Receiver clock is synchronized to the start bit, the data recovery can
begin. Data recovery process is almost similar to the clock recovery process. The data recovery logic
samples 16 times for each incoming bits for Normal mode and 8 times for Double Speed mode. And
uses sample 8, 9, and 10 to decide data value for Normal mode, samples 4, 5, and 6 for Double
Speed mode. If more than 2 samples have low levels, the received bit is considered to a logic 0 and
more than 2 samples have high levels, the received bit is considered to a logic 1. The data recovery
process is then repeated until a complete frame is received including the first stop bit. The decided bit
value is stored in the receive shift register in order. Note that the Receiver only uses the first stop bit
of a frame. Internally, after receiving the first stop bit, the Receiver is in idle state and waiting to find
start bit.
PS029502-0212
PRELIMINARY
103