Multichannel Selection Operation
11-73
Multichannel Buffered Serial Ports
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XMCM = 11b: In this mode, symmetric transmit and receive operation is
forced. Symmetric operation occurs when a device transmits and receives
on the same set of subframes. These subframes are determined by setting
RP(A/B)BLK. The elements in each of these subframes can then be en-
abled/selected using the RCER register for receive. The transmit side
uses the same blocks as the receive side (thus the value of X(P/A)BLK
does not matter). In this mode, all elements are disabled, so DR and DX
are in the high-impedance state. For receiving, a RBR-to-DRR copy oc-
curs only for those elements that are selected via RP(A/B)BLK and RCER.
If RINT were to be generated for every RBR-to-DRR copy, it would occur
as many times as the number of elements selected in RCER (and
not the
number of elements programmed in RFRLEN1). For transmitting, the
same subframe that is used for reception is used to maintain symmetry,
so the value XP(A/B)BLK does not matter. DXR is loaded, and DXR-to-
XSR copy occurs for all the elements that are enabled via RP(A/B)BLK .
However, DX is driven only for those elements that are selected via XCER.
The elements enabled in XCER can be either a subset of or the same as
those selected in RCER. Therefore, if XINTM = 00b, transmit interrupts to
the CPU would be generated the same number of times as the number of
elements selected in RCER (not XCER).
Figure 11–48 shows the activity on the McBSP pins for all of the preceding
XMCM values with the following conditions:
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(R/X)PHASE = 0: Single-phase frame for multichannel selection enabled
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FRLEN1 = 011b: 4-element frame
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WDLEN1 = Any valid serial element length
In the following illustrations, the arrows indicating the occurrence of events are
only sample indications.