Texas Instruments TRF7970A Manual page 21

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Figure 6-3. Nominal Start-up Sequence Using SPI With SS (MCU Controls EN2)
Figure 6-4. Nominal Start-up Sequence Using Parallel (MCU Controls EN2)
This start-up mode lasts until all of the regulators have settled and the 13.56-MHz oscillator has stabilized.
If the EN input is set high (EN = 1) by the MCU (or other system device), the reader stays active. If the EN
input is not set high (EN = 0) within 100 µs after the SYS_CLK output is switched from auxiliary clock (60
kHz) to high-frequency clock (derived from the crystal oscillator), the reader system returns to complete
Power-Down Mode 1. This option can be used to wake-up the reader system from complete Power Down
(PD Mode 1) by using a pushbutton switch or by sending a single pulse.
After the reader EN line is high, the other power modes are selected by control bits within the chip status
control register (0x00). The power mode options and states are listed in
When EN is set high (or on rising edge of EN2 and then confirmed by EN = 1) the supply regulators are
activated and the 13.56-MHz oscillator is started. When the supplies are settled and the oscillator
frequency is stable, the SYS_CLK output is switched from the auxiliary frequency of 60 kHz to the 13.56-
MHz frequency derived from the crystal oscillator. At this point, the reader is ready to communicate and
perform the required tasks. When this occurs, osc_ok (B6) of the RSSI Level and Oscillator Status register
is set. The MCU can then program the Chip Status Control register 0x00 and select the operation mode
by programming the additional registers.
• Standby Mode (bit 7 = 1 of register 0x00), the reader is capable of recovering to full operation in
100 µs.
• Mode 1 (active mode with RF output disabled, bit 5 = 0 and bit 1 = 0 of register 0x00) is a low power
mode which allows the reader to recover to full operation within 25 µs.
• Mode 2 (active mode with only the RF receiver active, bit 1 = 1 of register 0x00) can be used to
measure the external RF field (as described in RSSI measurements paragraph) if reader-to-reader
anticollision is implemented.
• Modes 3 and 4 (active modes with the entire RF section active, bit 5 = 1 of register 0x00) are the
normal modes used for normal transmit and receive operations.
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VIN
2 ms
SS
EN2
EN
VIN
EN2
EN
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5 ms
6 ms
5 ms
6 ms
Table
TRF7970A
TRF7970A
6-3.
Detailed Description 21