SPRUI30H November 2015 – May 2024 DRA745 , DRA746 , DRA750 , DRA756
The McASP features a digital loopback mode (DLB) that allows loopback test transfers in TDM mode between McASP transmitters and receivers within the same device. In loopback mode, the output of a transmit serializer is connected internally to the input of a receive serializer. Therefore, a receiver data can be checked against a transmitter data to ensure that the McASP settings are correct. Digital loopback mode applies to TDM mode only (2 to 32 slots in a frame). It does not apply to DIT mode (XMOD = 0x180) or burst mode (XMOD = 0).
Figure 24-132shows the basic logical connection of the serializers in loopback mode.
Two types of loopback connections are possible, selected by the ORD bit in the digital loopback control register - MCASP_LBCTL as follows:
User can choose in software (bit IOLBEN of the MCASP_LBCTL) between a McASP module internal loopback and a device I/O level loopback.
When a McASP internal loopback is selected (MCASP_LBCTL[4] IOLBEN=0b0 ), it is NOT necessary to configure MCASP_PFUNC and MCASP_PDIR registers for McASP pin settings. Nevertheless, data can be optionally made externally visible at the I/O pin of the transmit serializer, if the pin is configured as a McASP output pin by setting the corresponding MCASP_PFUNC bit to 0 (i.e. to function as McASP, not GPIO) and MCASP_PDIR bit to 1 (output).
When a device I/O level loopback is selected (MCASP_LBCTL[4] IOLBEN=0b1 ), the MCASP_PFUNC and MCASP_PDIR registers must be configured with the appropriate settings for all AXRn pins, according to ORD bit configuration.
In case of device I/O loopback, the connectivity is externally applied between device pads (i.e. reaching device I/O buffers ).
Hence, the corresponding padconfiguration registers must be appropriately configured in the device Control Module - CTRL_MODULE_CORE_PAD. For more details, see Pad Configuration Rregisters, in Control Module.
When In loopback mode, the transmit clock and frame sync are used by both the transmit and receive sections of the McASP. The transmit and receive sections operate synchronously. This is achieved by setting the MODE bitfield of the MCASP_LBCTL register to 0x1 and the ASYNC bit of the MCASP_ACLKXCTL register to 0b0.