How do I enable the tag antenna after DC power is applied to Monza X?

If you're not able to read the Monza X-based RFID tag with a reader once you've applied DC power to it, then the following information will help you.

Monza X is a UHF Gen 2/ISO 18000-6C compliant passive RFID integrated circuit that incorporates an I2C interface.  Although Monza X will operate in full passive mode when used to communicate to Gen 2 compliant RFID readers, DC power must be applied to DCI pin 4 and GND pin 8 of the IC to enable I2C communication.  The factory default for Monza X is disabled RF functionality when DC power is applied.

To enable RF functionality, the DCI_RF_EN. bit must be set.  This bit is found in byte 21 of Reserved memory and can only be set over I2C.

 Figure 1: I2C Interface Memory Map in a Word Wise format

RF Access Control

Monza® X-2K Dura provides three levels of control over RF access as follows:

1. Setting either or both the RF1_DIS or RF2_DIS bits in byte 21 of the NVM disables RF access on the corresponding RF port. These bits are accessible only to I2C, not RF.  The factory defaults are 0, enabling RF1 and RF2. 
2. Setting the DCI_RF_EN bit to 0 in byte 21 of the NVM inhibits all RF access when DCI voltage is present.  This takes precedence over the state of the RF1_DIS, RF2_DIS bit in #1 above.  This bit is also only accessible from I2C, not RF.  This bit is set by factory default to 0.  Thus, by default, there is RF access to RF1, RF2 when DCI voltage is NOT present and no RF access when DCI voltage is present. The specification V_RFON determines the DCI voltage that inhibits RF.
3. Setting the KILL bit 2 in byte 9 of the NVM will inhibit all RF access. This bit can be set from RF using a Gen2 KILL command or by writing from I2C. This is the normal mechanism for a reader to disable a chip. This KILL bit takes precedence over both #1 and #2 above.  I2C can re-write this bit back to 0 and thus reverse and RF KILL command. The factory default for KILL is 0.

The factories defaults are set so Monza® X-2K Dura operates like any other RFID tag when DCI voltage is not present. When DCI voltage is present the default behavior, through mechanism #2 above, is to inhibit all RF access. The KILL bit is always set to 0 at the factory by Gen2 definition.  In conventional RFID chips once this bit is set to 1 a chip is dead and can never be resurrected. Monza® X-2K Dura, having a hard wired I2C interface, allows un-doing the KILL operation from I2C.