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August ’01 Chapter 2. Electrical Description

- PTx16 H-Bit frequency of Antenna 1 without plucking

- PTx18 L-Bit frequency of Antenna 1 without plucking

- PTx28 L-Bit frequency of Antenna 2 without plucking

- PTx38 L-Bit frequency of Antenna 3 without plucking

2.11.1 Resonance Frequency Measurement

The first action in the trimming process is a measurement of the base resonant circuit frequen-

cy. For an optimum energy transfer the trimming should be done for a VCL of 4V, because

this voltage is necessary for a LF Response but is still below the limitation voltage. Therefore

VCL is supplied externally via the Test Interface (see Figure 8). During this test VCL and

VBM must be equal (VCL, VBAT = 4V).

After test number (18hex, 28hex, 38hex) is shifted into the IC (see Figure 24), the next clock

signal at TCLK triggers an oscillation at the addressed resonant circuit (RF1, RF2, RF3). The

TCLK signals should be short (tpluck), because during this time th RF pin is short circuit to

ground and a high current will flow from VCL via the coil.

This trigger action is further called 'pluck', because the action can be compared with the

plucking of a spring, which is fasten at one side (VCL) and pulled down to ground.

The oscillation fades away depending on the resonant circuit quality factor. Because the

TMS37122 uses a separate Clock Regenerator for this test, a clock will appear at TDAT im-

mediately if oscillation starts. Preferable at least the duration of 10 cycles should be mea-

sured. Then period duration of a single cycle can be calculated, which represents the

resonance frequency.

In case an additional period duration measurement is desired, the oscillation can be enhanced

once more with a further TCLK signal, after measurement is completed.

The measurement will be finished switching off TEN and VCL.

Important Note:

Do not measure at the RF pins during trimming process, because the capaci-

tance load will falsify the result. The same is the case if metal is near by the

antennas.

Texas Instruments TRIS TMS37122 User Manual

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