Rainbow Electronics AT75C220 - Manual (Page 80)

AT75C220
80
Interrupt Generation
Each status bit in US_CSR has a corresponding bit in
US_IER and US_IDR that controls the generation of inter-
rupts by asserting the USART interrupt line connected to
the AIC. US_IMR indicates the status of the corresponding
bits.
When a bit is set in US_CSR and the same bit is set in
US_IMR, the interrupt line is asserted.
Channel Modes
The USART can be programmed to operate in three differ-
ent test modes using the field CHMODE in US_MR.
Automatic echo mode allows bit-by-bit re-transmission.
When a bit is received on the RXD line, it is sent to the TXD
line. Programming the transmitter has no effect.
Local loopback mode allows the transmitted characters to
be received. TXD and RXD pins are not used and the out-
put of the transmitter is internally connected to the input of
the receiver. The RXD pin level has no effect and the TXD
pin is held high, as in idle state.
Remote loopback mode directly connects the RXD pin to
the TXD pin. The transmitter and the receiver are disabled
and have no effect. This mode allows bit-by-bit re-transmis-
sion.
Figure 21. Channel Modes
Receiver
Transmitter
Disabled
RXD
TXD
Receiver
Transmitter
Disabled
RXD
TXD
V
DD
Disabled
Receiver
Transmitter
Disabled
RXD
TXD
Disabled
Automatic Echo
Local Loopback
Remote Loopback V
DD