ST25R100 datasheet(21/86 Pages) STMICROELECTRONICS

21 / 86 page

The GPT can be started by sending the direct command start GPT. If this command is sent while the timer is

running, it is reset and restarted.

The GPT can be terminated using the direct command stop all activities.

Wake-up timer (WUT)

This timer is primarily used in the wake-up mode.

It can also be triggered by sending the direct command start WUT. When this command is sent, the RC oscillator,

which is used as the clock source for the wake-up timer, is started. When the timer expires, an IRQ with the I_wut

flag is sent.

As the RC oscillator is the clock source for the WUT, it can run during PD mode. The other timers, which are

based on the crystal oscillator, cannot be used in PD or WU mode.

Note:

The tolerance of WUT is defined by the tolerance of the RC oscillator.

5.10

Reader operation

To perform the function of the reader, the ST25R100 must be placed in RD mode as detailed on

Section 5.2: Power-on sequence. Once in RD mode, the mode (om<3:0>) and bitrates (tx_rate<1:0>,

rx_rate<1:0>) to be used must be defined, as well as all the needed configurations for the transmitter and receiver

paths. Both the transmitter (tx_en) and receiver (rx_en) shall be enabled.

Several NFC standards define a guard time, which requires that the reader field must be turned on for a certain

duration (for instance 5 ms for ISO14443A/B) before the first command is sent.

Recommended preparation and execution of a transceiver operation:

•

Execute the direct command stop all activities

•

Execute the direct command reset Rx gain

•

Configure the timers accordingly

•

Define the length of data to be transmitted (ntx<12:0> and nbtx<2:0>)

•

Write the bytes to be transmitted in the FIFO

•

Send the direct command transmit data

•

When all the data is transmitted, an IRQ is sent to inform the host that the transmission is finished (I_txe)

After the transmission is completed, the receiver automatically starts to observe the RFI inputs to detect a tag

response. In case it is enabled (sq_en, agc_en) the squelch and AGC are started, the framing block processes

the subcarrier signal from the receiver and fills the FIFO with data. When the reception is started an IRQ with the

flag I_rxs is signaled, and the data is placed in the FIFO. When reception is completed an IRQ is sent to the host

(I_rxe), and the FIFO status register 1 and FIFO status register 1 display the amount of data in the FIFO and

further information so the host can process it. When data frames longer than the FIFO are to be transmitted or

received, an additional step is needed.

During transmission, when the remaining number of bytes is lower than the FIFO water level, an IRQ due to the

FIFO water level (I_wl) is signaled. The host then shall add more data in the FIFO. This step shall be repeated the

number of times needed to transmit the full frame. When all the data is transmitted, an IRQ (I_txe) is sent to

inform the host that the transmission has finished.

Similarly during reception, when the FIFO is loaded with more data than the FIFO water level, an IRQ (I_wl) is

sent and the host shall retrieve the data from the FIFO. This step shall be repeated the number of times needed

to receive the full frame. When the reception is finished, an IRQ (I_rxe) is sent to the host and the remainder of

the data could be retrieved from the FIFO. FIFO water level is 128 bytes in both directions (transmission and

reception).

The diagrams below depict a few examples of transceiver operations.

ST25R100

Application information

DS14139 - Rev 5

page 21/86

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