Z85233 Datasheet, PDF (60/317 Pages) Zilog, Inc. – The Zilog SCC Serial Communication Controller

SCC™/ESCC™ User’s Manual
Interfacing the SCC/ESCC
2.6 TEST FUNCTIONS
The SCC contains two other features useful for diagnostic _{2.6.2 Auto Echo}
purposes, controlled by bits in WR14. They are Local
Loopback and Auto Echo.
Auto Echo is selected when bit D3 of WR14 is set to 1. In
this mode, the TxD pin is connected directly to the RxD pin,
2
2.6.1 Local Loopback
and the receiver input is connected to the RxD pin. In this
mode, the /CTS pin is ignored as a transmitter enable and
Local Loopback is selected when WR14 bit D4 is set to 1. _{the output of the transmitter does not connect to anything.}
In this mode, the output of the transmitter is internally con- _{If both the Local Loopback and Auto Echo bits are set to 1,}
nected to the input of the receiver. At the same time, the _{the Auto Echo mode is selected, but both the /CTS pin and}
TxD pin remains connected to the transmitter. In this _{/DCD pin are ignored as auto enables. This should not be}
mode, the /DCD pin is ignored as a receive enable and the _{considered a normal operating mode (Figure 2-36).}
/CTS pin is ignored as a transmitter enable even if the Auto
Enable mode has been selected. Note that the DPLL input
is connected to the RxD pin, not to the input of the receiver.
This precludes the use of the DPLL in Local Loopback. Lo-
/DCD
cal Loopback is shown schematically in Figure 2-35.
Rx Enable
/DCD
Rx Enable
RxD
Receiver
RxD
NC
TxD
Receiver
Transmitter
TxD
/CTS
NC
Transmitter
Tx Enable
/CTS
Tx Enable
Local Loop Back
Figure 2-35. Local Loopback
Auto Echo
Figure 2-36. Auto Echo
UM010901-0601
2-41