Intel® Arria® 10 Hard Processor System Technical Reference Manual

ID 683711
Date 1/10/2023
Public

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20.4.2.2. Transmit and Receive FIFO Buffers

There are two 16‑bit FIFO buffers, a transmit FIFO buffer and a receive FIFO buffer, with a depth of 256. Data frames that are less than 16 bits in size must be right‑justified when written into the transmit FIFO buffer. The shift control logic automatically right‑justifies receive data in the receive FIFO buffer. †

Each data entry in the FIFO buffers contains a single data frame. It is impossible to store multiple data frames in a single FIFO buffer location; for example, you may not store two 8‑bit data frames in a single FIFO buffer location. If an 8‑bit data frame is required, the upper 8‑bits of the FIFO buffer entry are ignored or unused when the serial shifter transmits the data. †

The transmit and receive FIFO buffers are cleared when the SPI controller is disabled (SSIENR=0) or reset.

The transmit FIFO buffer is loaded by write commands to the SPI data register (DR). Data are popped (removed) from the transmit FIFO buffer by the shift control logic into the transmit shift register. The transmit FIFO buffer generates a transmit FIFO empty interrupt request when the number of entries in the FIFO buffer is less than or equal to the FIFO buffer threshold value. The threshold value, set through the register TXFTLR, determines the level of FIFO buffer entries at which an interrupt is generated. The threshold value allows you to provide early indication to the processor that the transmit FIFO buffer is nearly empty. A Transmit FIFO Overflow Interrupt is generated if you attempt to write data into an already full transmit FIFO buffer. †

Data are popped from the receive FIFO buffer by read commands to the SPI data register (DR). The receive FIFO buffer is loaded from the receive shift register by the shift control logic. The receive FIFO buffer generates a receive FIFO full interrupt request when the number of entries in the FIFO buffer is greater than or equal to the FIFO buffer threshold value plus one. The threshold value, set through register RXFTLR, determines the level of FIFO buffer entries at which an interrupt is generated. †

The threshold value allows you to provide early indication to the processor that the receive FIFO buffer is nearly full. A Receive FIFO Overrun Interrupt is generated when the receive shift logic attempts to load data into a completely full receive FIFO buffer. However, the newly received data are lost. A Receive FIFO Underflow Interrupt is generated if you attempt to read from an empty receive FIFO buffer. This alerts the processor that the read data are invalid. †