CLKX
DX
FSX
For 0-bit delay:
Next frame-sync pulse
here or later is OK.
For 1-bit delay:
Next frame-sync pulse
here or later is OK.
For 2-bit delay:
Next frame-sync pulse
here or later is OK.
Last bit of
current frame
Earliest possible time
to begin transfer of
next frame
Preliminary
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Architecture
Figure 11-18. Proper Positioning of Transmit Frame-Sync Pulses
11.2.3.6 Overflow in the Transmitter
The McBSP indicates a transmitter overflow condition by setting the XOVFLSTAT bit in
IRQSTATUS[11] register. This error occurs when DMA controller or CPU write data to a full transmit
buffer (this may happen only if the CPU or DMA controller does not respect the DMA length, does not
wait for DMA request or does not check the buffer status before writing data. According to the
IRQENABLE register settings this condition can generate the COMMONIRQ line to be asserted low.
Writing 1 to the corresponding bit in status register will clear the interrupt.
11.2.4 McBSP DMA Configuration
The McBSP receive and transmit data DMA requests are active after the receive RRST and transmit
XRST are released. After reset the default DMA threshold (and length) is one.
The receive and transmit DMA requests can be individually disabled by setting zero the RDMAEN,
XDMAEN bits in RCCR_REG respectively XCCR_REG. When disabling the DMA the DMA request line
is deasserted even if a DMA transfer is pending and the DMA state machine is not reset.
The DMA threshold and length configuration is done through THRSH1_REG and THRSH2_REG
registers as follows:
•
(THRS 1) value represents the required receive DMA request length (the length of the
transfer is the same as the threshold value plus one). As long as the RB occupied locations level is
above or equal to the THRSH1_REG value plus one, the DMA request will be asserted. After
transferring the configured (THRS 1) number of words, the receive DMA request
(McBSP.REVNT) will be deasserted and reasserted as soon as the conditions are met again.
•
(THRS 1) value represents the required transmit DMA request length (the length of the
transfer is the same as the threshold value plus one). As long as the XB free locations level is
above or equal to the THRSH2_REG value plus one the DMA request will be asserted. After
transferring the configured (THRS 1) number of words, the transmit DMA request
(McBSP.XEVNT) will be deasserted and reasserted as soon as the conditions are met again.
Note that the CPU may decide not to use the DMA to transfer the data. In this case the DMA must be
disabled (or the DMA request can be ignored by CPU) and the common interrupt line can be used. The
RRDY bit for receive and XRDY bit for transmit will indicate when the threshold values are reached.
Also, by reading the receive buffer status RBUFFSTAT_REG register and transmit buffer status
XBUFFSTAT_REG register the CPU may decide to transfer data even if the threshold is not reached.
This mechanism is useful on the last transfer on receive side when the threshold value is bigger than
the occupied locations inside the receive buffer and the CPU needs to read this data. Since no interrupt
or DMA request is asserted the only option in this case is to read the receive buffer status register
value and to transfer the remaining data without using the DMA or interrupt indication.
1143
SPRUGX9 – 15 April 2011
Multichannel Buffered Serial Port (McBSP)
© 2011, Texas Instruments Incorporated
Содержание TMS320C6A816 Series
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