Rainbow Electronics AT75C220 - Manual (Page 77)

AT75C220
77
Receiver
Asynchronous Receiver
The USART is configured for asynchronous operation
when SYNC = 0 (bit 7 of US_MR). In asynchronous mode,
the USART detects the start of a received character by
sampling the RXD signal until it detects a valid start bit. A
low level (space) on RXD is interpreted as a valid start bit if
it is detected for more than seven cycles of the sampling
clock, which is 16 times the baud rate. Hence, a space
which is longer than 7/16 of the bit period is detected as a
valid start bit. A space which is 7/16 of a bit period or
shorter is ignored and the receiver continues to wait for a
valid start bit.
When a valid start bit has been detected, the receiver sam-
ples the RXD at the theoretical mid-point of each bit. It is
assumed that each bit lasts 16 cycles of the sampling clock
(1-bit period) so the sampling point is eight cycles (0.5-bit
periods) after the start of the bit. The first sampling point is
therefore 24 cycles (1.5-bit periods) after the falling edge of
the start bit was detected. Each subsequent bit is sampled
16 cycles (1-bit period) after the previous one.
Figure 17. Asynchronous Mode: Start Bit Detection
Figure 18. Asynchronous Mode: Character Reception
Synchronous Receiver
When configured for synchronous operation (SYNC = 1),
the receiver samples the RXD signal on each rising edge of
the baud rate clock. If a low level is detected, it is consid-
ered as a start. Data bits, parity bit and stop bit are sam-
pled and the receiver waits for the next start bit. See the
example in Figure 19.
Receiver Ready
When a complete character is received, it is transferred to
the US_RHR and the RXRDY status bit in US_CSR is set.
If US_RHR has not been read since the last transfer, the
OVRE status bit in US_CSR is set.
Parity Error
Each time a character is received, the receiver calculates
the parity of the received data bits in accordance with the
field PAR in US_MR. It then compares the result with the
received parity bit. If different, the parity error bit PARE in
US_CSR is set.
Framing Error
If a character is received with a stop bit at low level and
with at least one data bit at high level, a framing error is
generated. This sets FRAME in US_CSR.
16 x Baud
Rate Clock
RXD
True Start
Detection
D0
Sampling
D0 D1 D2 D3 D4 D5 D6 D7
RXD
True Start Detection
Sampling
Parity Bit
Stop Bit
Example: 8-bit, parity enabled 1 stop
1-bit
period
0.5-bit
periods