S2-LPS datasheet(33/73 Pages) STMICROELECTRONICS

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4.4.6.3

Data FIFO

In the S2-LPS there are two data FIFOs, a TX FIFO for data to be transmitted and an RX FIFO for the received

data both of 128 bytes. The SPI interface is used to read from the RX FIFO and write to the TX FIFO starting from

the address 0xFF.

Figure 11. Threshold in FIFO

The TX FIFO has two programmable thresholds (see figure above). An interrupt event occurs when the data in

the TX FIFO reaches any of these thresholds. The first threshold is the “FIFO Almost Full” threshold, TX_AF_THR

registers. The value in this field corresponds to the desired threshold value in number of bytes + 2. When empty

locations (free) amount inside the TX FIFO reaches this threshold limit, an interrupt to the MCU is generated so

it can send a TX command to transmit the contents of the TX FIFO. The second threshold for TX is the “FIFO

Almost Empty” threshold, TX_AE_THR register. When the data being shifted out of the TX FIFO reaches the

Almost Empty threshold, an interrupt will be generated also. The MCU could to switch out of TX mode or fill new

data into the TX FIFO.

The RX FIFO has two programmable thresholds (see figure above). The first threshold is the “FIFO Almost

Full” threshold, RX_AF_THR0 registers. The value in this register corresponds to the desired threshold value in

number of bytes. When empty locations (free) amount inside the RX FIFO reaches this threshold limit, an interrupt

will be generated to the MCU. The MCU should then start to read the data from the RX FIFO. The second

threshold for RX is the “FIFO Almost Empty” threshold, RX_AE_THR register. When the data being shifted out

of the RX FIFO reaches the Almost Empty threshold, an interrupt will be generated also. The MCU will need to

switch on RX mode to fill with new data the RX FIFO or stop to read after the number of byte indicated by the

RX_AE_THR register.

In order to enable the RX_FIFO thresholds interrupts, the bit FIFO_GPIO_OUT_MUX_SEL (PROTOCOL2

set to 0.

The FIFO controller detects overflow or underflow in the RX FIFO and overflow or underflow in the TX FIFO. It

is the responsibility of the MCU to avoid TX FIFO overflow since the MCU only can decide to writing on the TX

FIFO. A TX FIFO overflow results in an error in the TX FIFO content, while an underflow results in the continuous

transmission of the last byte stored in the TX FIFO. Likewise, when reading the RX FIFO the MCU must avoid

reading the RX FIFO after its empty condition is reached, since a RX FIFO underflow will result in an error in the

data read out of the RX FIFO.

When an overflow or an underflow is detected, the MCU has to issue a SABORT and a FLUSHTXFIFO/

FLUSHRXFIFO command before resuming the normal transceiver activity. For each FIFO, when one of these

errors is detected an interrupt is generated to the MCU.

The TX FIFO may be flushed by issuing a FLUSHTXFIFO command (see Table 42. Commands). Similarly, a

FLUSHRXFIFO command flushes the RX FIFO.

S2-LPS

Digital modulator

DS13831 - Rev 1

page 33/73

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