ST25R200 datasheet(20/88 Pages) STMICROELECTRONICS

20 / 88 page

A FIFO underflow is set when data was read from an empty FIFO.

When this bit is set during reception, the host read more data than was actually received.

When an underflow happens during transmission, it means that the controller has failed to provide the quantity of

data defined in the number of transmitted bytes registers on time.

FIFO pointers and FIFO status are reset at the start of each data reception (I_rxs). They are also reset at power-

up and by commands set default and clear FIFO. Reading out data from empty/cleared FIFO shows data as

zeros.

5.9

Timers

Several timers are integrated in the device to eliminate the need to runtime critical tasks with counters/timers on

the host side, sampling system integration and improve its portability.

Each timer has one or more associated configuration registers in which the timeout duration and different

operating modes are defined. These configuration registers have to be set while the corresponding timer is not

running. Any modification of timer configuration while the timer is active may result in unpredictable behavior.

All timers can be stopped by the stop all activities direct command.

Mask receive timer (MRT)

This timer blocks the receiver and reception process in framing logic by keeping the rx_on signal low after the end

of transmission during the time that the tag reply is not expected.

While the mask receive timer is running the squelch can be automatically turned on.

The MRT does not produce an IRQ.

The MRT timeout is configured in the MRT SQT configuration register and MRT registers. It is automatically

started at the end of data transmission (at the end of EOF). In case MRT = 0 the timer does not run at the end of

transmission and the receiver remains blocked.

Additionally, the MRT can be triggered by starting the MRT direct command.

No-response timer (NRT)

The NRT is intended to observe whether a Tag/Listener response was detected within a configured time frame,

starting from the end of data transmission.

The IRQ with I_nre flag is triggered at the NRT timeout.

The NRT is configured by writing the NRT1/2 and NRT GPT configuration registers.

The NRT is started at the end of data transmission (at the end of EOF).

Bit nrt_step configures the time step of the no-response timer. Two steps are available, 64/fc (4.72 μs), which

covers range up to 309 ms and 4096/fc, which covers range up to 19.8 s.

NRT supports two operating modes controlled by bit nrt_emd:

•

When nrt_emd=0 (default mode), the IRQ is produced in case the NRT expires before a start of a Tag/

Listener reply is detected. Additionally, the rx_on is set low to disable the receiver.

In the opposite case, when the start of a tag reply is detected before the timeout, the timer is stopped, and

no IRQ is produced.

•

When nrt_emd=1 the NRT unconditionally produces an IRQ when it expires. This means that IRQ is

independent from whether or not a tag reply was detected.

When a tag reply is being processed during a timeout, no other action is taken and the reply is normally

received.

In the opposite case, when no tag response is being processed additionally the receiver is disabled.

Additionally, the NRT can be started using the direct command start NRT. The intention of this command is to

extend the NRT timeout. In case this command is sent while the timer is running, it is reset and restarted.

The NRT can be terminated using the direct command stop NRT or stop all activities. The timer is terminated and

no IRQ is sent. It is expected to be used when nrt_emd=1, where an incoming reception does not stop the NRT.

General purpose timer (GPT)

The GPT can be used for multiple purposes, as it allows it to be triggered manually or by several other trigger

sources. It can be used to measure the duration of the reception process (triggering by start of reception, after

SOF), or to time out the reader frame after the tag’s reception (triggered by end of reception, after EOF).

ST25R200

Application information

DS13658 - Rev 2

page 20/88

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