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SAB 82532/SAF 82532
Asynchronous Serial Mode
Semiconductor Group
73
07.96
Receiver
For some applications it is desirable to provide means of out-of-band flow control to
indicate to the far end transmitter that the local receiver’s buffer is getting full.
This flow control can be used between two DTEs as shown in
figure 33
and between a
DTE and a DCE (MODEM) as shown in
figure 34
that supports this kind of bi-directional
flow control.
Setting
MODE:FRTS = ‘1’
and MODE:RTS = ‘0’ invokes this out-of-band flow control for
the receiver. When the shadow part of RFIFO has reached a set threshold of 28 bytes,
the RTS signal is forced inactive (HIGH). When the shadow part of the RFIFO is empty,
the RTS is re-asserted (‘LOW’). Note that the data is immediately transferred from the
shadow RFIFO to the user accessible RFIFO (as long as there is space available). So
when the shadow RFIFO reaches 28 bytes threshold, there is 4 more byte storage
available before overflow can occur. This allows sufficient time for the far end transmitter
to react to the change in the RTS signal and stop sending more data.
Figure 33
shows the connection between two ESCC2 Version 3.2 devices as DTEs. The
RTS
A
of DTE
A
(ESCC2) feeds the CTS
B
input of the second DTE
B
(another ESCC2).
For example while DTE
A
is receiving data and its receiver RFIFO threshold is reached,
the RTS
A
signal goes in-active HIGH forcing the CTS
B
to become in-active indicating to
DTE
B
to stop transmitting. Both DTE devices should also be using the CTS signal to flow
control their transmitters. When the shadow RFIFO in DTE
A
is cleared the RTS
A
goes
active ‘LOW’ and this signals the far end to resume transmission. Data flow control from
DTE
B
to DTE
A
works in a similar way.
Figure 33
Out-of-Band DTE-DTE Bi-directional Flow Control