MCP451 datasheet(30/50 Pages) MICROCHIP

30 / 50 page

MCRF450/451/452/455

DS40232H-page 30

 2003 Microchip Technology Inc.

6.2.3.5

Calibration Of Time Reference For

Decoding

The device uses TCP to match its internal decoder

timing to the Interrogator timing. The Interrogator

transmits the timing pulses at the start of all

commands and at least every 17 symbols. The TCP

uses a code violation of the 1-of-16 PPM signal

consisting of three gap pulses within one symbol. The

first gap pulse is located at position 0, the second gap

pulse at position 6 and the third at position 14 of the

symbol. The time period between the last two gap

pulses is used to calibrate the device’s timing for

decoding. Figure 6-10 shows the calibration pulses

for Normal Speed mode. The waveform of the gap

pulses is the same as the 1-of-16 PPM signal, as

shown in Figure 6-2. For Fast Speed mode, the gap

positions are the same. PW

PPM_F is the gap pulse

width and SW

PPM_F is the symbol width of the Fast

mode.

FIGURE 6-10:

CALIBRATION PULSES FOR NORMAK SPEED MODE

6.2.3.6

Calculation Of Matching Code

When the Interrogator receives the FR response from

a device, it sends an MC to select the device. The MC

is sent during the device’s listening window. There are

two different types of matching codes: MC1 and MC2.

Both MC1 and MC2 are used in the detection loop.

MC2 is used in the reactivation loop, as detailed in

Figure 6-1. The MC1 command is used to send the

device to the sleeping loop, while MC2 is used to send

the device to the processing loop.

The MC is an 8-bit “match” of tag ID followed by 4-bit

matching code type and parity bit such that:

Matching code (12 bits) = “match (8 bits of tag

ID)” + matching code type (3 bits)+ parity (1-bit)

The matching code type and parity bit is bit-wise

structured as follows:

•MC1: 010P

•MC2: 100P

where P represents the parity bit of all match bits (8

bits) plus the MC type (3 bits).

The “match” part of the MC is 8 bits of the 32-bit Tag ID.

The Interrogator selects the 8 bits from the 32-bit Tag

ID by calculating the bit range of the Tag ID.

Equation 6-2 shows the equation for selecting the bit

range using the transmission counter (TC). Both the

0.5

1000

2000

2500

3000

(ms)

Gap Pulse Width

Calibration Time Reference

Position 0

Position 6

Position 14

500

1500

500

(PWPPM_N)

Symbol Width

(SWPPM_N)

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