Automatic transmission of dummy bits in bus master
Various embodiments are disclosed for automatic transmission of dummy bits in a serial bus master. The disclosed embodiments allow a single DMA descriptor to be fetched from memory for the reception of a specified amount of data. Dummy bits can be located or generated in the serial bus master either as a user configurable value or a default value. Logic in the serial bus master initiates a data transfer by writing a count value representing an amount of data to be received to a count register in the serial bus master. The single DMA descriptor is then configured to handle the internal transfer of bits received by the serial bus master from a serial bus slave and the DMA controller is enabled. When data transfer is initiated, the serial bus master starts sending dummy bits to the serial bus slave and receiving data bits from the serial bus slave.
1. A method comprising:
obtaining, by a direct memory access (DMA) controller (DMAC), a single DMA descriptor from a system memory for a data transfer over a serial bus, the serial bus coupled to a serial peripheral interface (SPI) master and a SPI slave, the single DMA descriptor including a count value for tracking a number of data unit transfers received by the SPI master from the SPI slave over the serial bus during the data transfer;
automatically and without accessing the system memory for another DMA descriptor for the data transfer:
transferring, by the SPI master over the serial bus, a first set of dummy bits to the SPI slave coupled to the serial bus;
receiving, by the SPI master over the serial bus, a first set of data bits from the SPI slave;
storing, by the SPI master, the first set of data bits in a buffer;
adjusting, by the SPI master, the count value for tracking the number of data unit transfers received from the SPI slave over the serial bus;
signaling, by the SPI master to the DMAC, that the first set of data bits have been received;
transferring, by the SPI master over the serial bus, a second set of dummy bits to the SPI slave;
receiving, by the SPI master over the serial bus, a second set of data bits from the SPI slave;
storing, by the SPI master, the second set of data bits in the buffer;
adjusting, by the SPI master, the count value for tracking the number of data unit transfers over the serial bus; and
signaling, by the SPI master to the DMAC, that the second set of data bits have been received.
2. A microcontroller unit comprising:
system memory;
a serial bus;
a serial peripheral interface (SPI) master coupled to the serial bus;
an SPI slave coupled to the serial bus;
a direct memory access (DMA) controller (DMAC) coupled to the system memory and the SPI master, the DMAC configured to obtain a single DMA descriptor from the system memory for a data transfer, the single DMA descriptor including a count value for tracking a number of data unit transfers received from the SPI slave over the serial bus during the data transfer;
the SPI master configured to automatically and without accessing the system memory for another DMA descriptor for the data transfer:
transfer over the serial bus a first set of dummy bits to the SPI slave;
receive over the serial bus a first set of data bits from the SPI slave;
store the first set of data bits in a buffer;
adjust the count value;
signal to the DMAC, that the first set of data bits have been received;
transfer to the SPI slave a second set of dummy bits;
receive over the serial bus a second set of data bits from the SPI slave;
store the second set of data bits in the buffer;
adjust the count value; and
signal to the DMAC that the second set of data bits have been received.