Specifications

5.4. HARDWARE

samples

Thus, a faster device, or one with more memory was required. Mr Andrew

Markham recommended Microchip’s PIC 18F series and although we had no

experience with these devices, they had been succes sfully used in upgrading

the existing system on the island.

Second design based on PIC 18F452

The PIC 18F452 has an on-board divide by four counter as part of its

Capture, Compare, PWM (CCP) peripheral. With 1.5kB of RAM it should

be possible to capture all data and perform decoding and CRC checking oﬀ-

line. Furthermore, the device has an on-board 4x PLL for increasing the bus

frequency. An 8MHz crystal was used, which gave the option of clocking the

device up to 32MHz simply by enabling the PLL.

Unfortunately, 134.2kHz is not directly obtainable from the 8MHz crystal.

Thus, a form of direct digital synthesis is used. By changing the timer

modulus from 60 (giving a 133.33kHz signal) to 59 (giving a 135.59kHz signal)

on every third cycle, an average frequency of 134.09kHz is achieved. Figure

5.12 in the measurements section shows an FFT plot of the positive half of

the spectrum of the resulting transmitted signal.

Possible Optimisation on PIC 18F4620

After completing experimental measurements (see Section 5.6 for further

details), it became clear that it was not possible to reliably detect the

transponder in all orientations using a single antenna. The proposed solution

is thus to use additional receiving antennae, placed in diﬀerent orientations.

A single transmitter pulse will be sent and both antennae will listen for a

response from the transponder. The charge pulses to both antennae will be

synchronised and then the received signal from each decoded independently.

Ideally, a single processor could be used to decode the received signals

from both antennae. Thus, double the RAM would be required to store the

captured data from the extra channel for oﬀ-line decoding. Furthermore,

if both CCP modules are clocked from the same timer3 module (one CCP

128bits, 4 edges per bit gives 512 samples