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ST25R3916B Datasheet(PDF) 24 Page - STMicroelectronics |
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ST25R3916B Datasheet(HTML) 24 Page - STMicroelectronics |
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24 / 165 page  Application information ST25R3916B ST25R3917B 24/165 DS13541 Rev 7 4.2.1 Transmitter The transmitter contains two identical push-pull driver blocks connected to pins RFO1 and RFO2. These drivers are differentially driving the external antenna LC tank. It is also possible to operate only one of the two drivers by setting the IO configuration register 1 bit single and selecting which RFO/RFI to be use on bit rfo2. Output resistance Each driver is composed of eight segments having binary weighted output resistance. The MSB segment typical ON resistance is 4 Ω. When all segments are turned on, the output resistance is typically 2 Ω. Usually all segments are turned on to define the normal transmission (non-modulated) level. It is also possible to switch off certain MSB segments when driving the non-modulated level to drive the circuitry with a higher impedance driver. The bits d_res<3:0> in the TX driver register define the resistance during the normal transmission. The default setting is minimum available resistance. When using the single driver mode, the number and therefore the cost of the antenna LC tank components is halved, but also the output power is reduced. In single mode it is possible to connect two antenna LC tanks to the two RFO outputs and multiplex between them by controlling the IO configuration register 1 bit rfo2. To transmit data, the transmitter output level needs to be modulated. AM and OOK modulation principles are supported. The type of modulation is defined by setting bit tr_am in the Mode definition register. Driver TX modulation During the OOK modulation (e.g. for ISO14443A) the transmitter drivers stop driving the carrier frequency. As a consequence the amplitude of the antenna LC tank oscillation decays, the time constant of the decay is defined with the LC tank Q factor. AM modulation (for example ISO14443B) is done via an additional regulator providing the supply voltage VDD_AM, used as the driver supply voltage during the modulation state. The AM modulation level is set by am_mod3:0 bits in the TX driver register. AM modulation has to be manually enabled and the level to be set correctly for the following protocols: • ISO14443B • FeliCa • ISO15693 (if not OOK) • NFCIP-1 212 and 424 kb/s initiator or active target. Depending on the applicable standard the modulation index is set in a range between 0 and 82% in the TX driver register. Passive load modulation The ST25R3916B and ST25R3917B enable passive load modulation using two different methods • Internal driver load modulation • Load modulation with an external MOS transistor and a diode that directly loads the antenna circuit |
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