Processor Users Manual

MOTOROLA Chapter 19. Serial Communications Controllers (SCCs) 19-21
Part IV. Communications Processor Module
Figure 19-12. Using CD to Control Synchronous Protocol Reception
If CD is programmed to envelope the data, it must remain asserted during frame
transmission or a CD
lost error occurs. Negation of CD terminates reception. If
GSMR_H[CDS] is zero, CD
must be sampled by the SCC before a CD lost error is
recognized; otherwise, the negation of CD
immediately causes the CD lost condition.
If GSMR_H[CDS] is set, all CD
transitions must occur while the Rx clock is low.
19.3.5.2 Asynchronous Protocols
In asynchronous protocols, RTS is asserted when SCC data is loaded into the Tx FIFO and
a falling Tx clock occurs. CD
and CTS can be used to control reception and transmission
in the same manner as the synchronous protocols. The Þrst bit sent in an asynchronous
protocol is the start bit of the Þrst character. In addition, the UART protocol has an option
for CTS
ßow control as described in Chapter 20, ÒSCC UART Mode.Ó
¥ If CTS
is already asserted when RTS is asserted, transmission begins in two
additional bit times.
¥ If CTS
is not already asserted when RTS is asserted and GSMR_H[CTSS] = 0,
transmission begins in three additional bit times.
¥ If CTS
is not already asserted when RTS is asserted and GSMR_H[CTSS] = 1,
transmission begins in two additional bit times.
1. GSMR_H[CDS] = 0. CDP=0.
RCLK
First Bit of Frame Data
NOTE:
CD Sampled Low Here
RCLK
CD Sampled High Here
Last Bit of Frame Data
2. If CD is negated prior to the last bit of the receive frame, CD lost is signaled in the frame BD.
3. If CDP=1, CD lost cannot occur and CD negation has no effect on reception.
1. GSMR_H[CDS] = 1. CDP=0.
NOTE
:
2. If CD is negated prior to the last bit of the receive frame, CD lost is signaled in the frame BD.
3. If CDP=1, CD lost cannot occur and CD negation has no effect on reception.
Last Bit of Frame DataFirst Bit of Frame Data
CD Assertion Immediately
Gates Reception
CD Negation Immediately
Halts Reception
RXD
(Input)
CD
(Input)
CD
(Input)
RXD
(Input)