Rainbow Electronics AT75C220 - Manual (Page 78)

AT75C220
78
Time-out
This function allows an idle condition on the RXD line to be
detected. The maximum delay for which the USART should
wait for a new character to arrive while the RXD line is inac-
tive (high level) is programmed in US_RTOR. When this
register is set to 0, no time-out is detected. Otherwise, the
receiver waits for a first character and then initializes a
counter which is decremented at each bit period and
reloaded at each byte reception. When the counter reaches
0, the TIMEOUT bit in US_CSR is set. The user can restart
the wait for a first character with the STTTO (Start Time-
out) bit in US_CR.
Calculation of time-out duration:
Figure 19. Synchronous Mode: Character Transmission
Transmitter
The transmitter has the same behavior in both synchro-
nous and asynchronous operating modes. Start bit, data
bits, parity bit and stop bits are serially shifted, lowest sig-
nificant bit first, on the falling edge of the serial clock. See
the example in Figure 20.
The number of data bits is selected in the CHRL field in
US_MR.
The parity bit is set according to the PAR field in US_MR.
The number of stop bits is selected in the NBSTOP field in
US_MR.
When a character is written to US_THR, it is transferred to
the Shift Register as soon as it is empty. When the transfer
occurs, the TXRDY bit in US_CSR is set until a new char-
acter is written to US_THR. If the Transmit Shift Register
and US_ THR are both emp ty, the TXEMPTY bit in
US_CSR is set.
Time-guard
The time-guard function allows the transmitter to insert an
idle state on the TXD line between two characters. The
duration of the idle state is programmed in US_TTGR.
When this register is set to zero, no time-guard is gener-
ated. Otherwise, the transmitter holds a high level on TXD
after each transmitted byte during the number of bit periods
programmed in US_TTGR.
Multi-drop Mode
When the field PAR in US_MR equals 11X (binary value),
the USART is configured to run in multi-drop mode. In this
case, the parity error bit PARE in US_CSR is set when data
is detected with a parity bit set to identify an address byte.
PARE is cleared with the Reset Status Bits Command
(RSTSTA) in US_CR. If the parity bit is detected low, identi-
fying a data byte, PARE is not set.
The transmitter sends an address byte (parity bit set) when
a Send Address Command (SENDA) is written to US_CR.
In this case, the next byte written to US_THR will be trans-
mitted as an address. After this, any byte transmitted will
have the parity bit cleared.
Duration Value 4
×
Bit Period
×
=
D0 D1 D2 D3 D4 D5 D6 D7
RXD
True Start Detection
Sampling
Parity Bit
Stop Bit
Example: 8-bit, parity enabled 1 stop
SCK
Idle state duration
between two characters
=
Time-guard
value
x
Bit
period