TMS320C5517: The transmitted data from McBSP on TMS320C5517

Hi Nomo-san,

Hi,

Couple questions:

When McBSP is not transmitting data in this system, does the software stop the McBSP clocks?

Or alternately service the McBSP interrupts/DMA events by writing dummy data to the transmit register (DXR)?

If no data is written to DXR for multiple frames, then the underflow condition will occur and the McBSP will behave as described in 8.2.6.5.4 Transmit Empty: XEMPTY.

And your answers:

1.

During an underflow condition, the transmitter continues to transmit the old data in DXR for every new frame sync signal FSX (generated by an external device, or by the internal sample rate generator) until a new element is loaded into DXR by the CPU or the DMA controller

If DXR is loaded by the CPU or the DMA controller before FSX goes active, a valid DXR-to-XSR transfer occurs. This allows for the first element of the first frame to be valid even before the transmit frame sync pulse is generated or detected. Alternatively, if a transmit frame sync is detected before DXR is loaded, 0s are output on DX.

2.

The reset is described in 8.2.10 Resetting the Serial Port: RRST, XRST, GRST, and RESET:

Device reset or McBSP reset: When the McBSP is reset by device reset or McBSP reset, the state machine is reset to its initial state. All counters and status bits are reset. This includes the receive status bits RFULL, RRDY, and RSYNCERR, and the transmit status bits XEMPTY, XRDY, and XSYNCERR in the serial port control register (SPCR).

The serial port can be reset in the following two ways:

• Device reset (RESET pin is low) places the receiver, the transmitter, and the sample rate generator in reset. When the device reset is removed (RESET = 1), FRST = GRST = RRST = XRST = 0 in SPCR, keeping the entire serial port in the reset state.

• The serial port transmitter and receiver can be independently reset by the XRST and RRST bits in SPCR. The sample rate generator is reset by the GRST bit in SPCR.

Table 8-18 shows the state of the McBSP pins when the serial port is reset by these methods

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Nomo said:

In the paragraph, "DXR is already serviced" is described.
Could you let me know how to know the "DXR is already serviced" status?

It refers to the DMA event, interrupt, or polling software that causes new data to be written into the data transmit register (DXR). If the software writes to DXR at the start of a frame, then it has the entire period of the frame to perform the DXR-to-XSR transfer so the new data will be ready to shift out. That is plenty of time.

Regards,
Mark