Texas Instruments OMAP5912 Reference Manual page 1679

Multichannel Selection Modes
5.7.1 Disabling/Enabling Versus Masking/Unmasking
68 Multichannel Buffered Serial Ports (McBSPs)
As an example of how the McBSP behaves in a transmit multichannel
selection mode, suppose that XMCM = 01b (all channels disabled unless
individually enabled) and that you have enabled only channels 0, 15, and 39.
Suppose also that the frame length is 40. The McBSP:
1) Shifts data to the DX pin in channel 0.
2) Places the DX pin in the high impedance state in channels 1-14.
3) Shifts data to the DX pin in channel 15.
4) Places the DX pin in the high impedance state in channels 16-38.
5) Shifts data to the DX pin in channel 39.
For transmission, a channel can be:
Enabled and unmasked (transmission can begin and can be completed).
-
Enabled but masked (transmission can begin but cannot be completed).
-
Disabled (transmission cannot occur).
-
The following definitions explain the channel control options:
Enabled channel
Masked channel
Disabled channel
Unmasked channel
A channel that can begin transmission by passing
data from the data transmit register(s) (DXR(s)) to
the transmit shift registers (XSR(s)).
A channel that cannot complete transmission. The
DX pin is held in the high impedance state; data
cannot be shifted out on the DX pin.
In systems where symmetric transmit and receive
provides software benefits, this feature allows
transmit channels to be disabled on a shared serial
bus. A similar feature is not needed for reception
because multiple receptions cannot cause serial bus
contention.
A channel that is not enabled. A disabled channel is
also masked.
Because no DXR-to-XSR copy occurs, the XRDY bit
of SPCR2 is not set. Therefore, no DMA
synchronization event (XEVT) is generated, and if
the transmit interrupt mode depends on XRDY
(XINTM = 00b in SPCR2), no interrupt is generated.
The XEMPTY bit of SPCR2 is not affected.
A channel that is not masked. Data in the XSR(s) is
shifted out on the DX pin.
SPRU762B