Specifications

5.4. HARDWARE
channel c annot be allowed to reset the timer upon receipt of an edge as it
would interfere with the results of the second channel), each read will need to
be 16bits long rather than 8bits in order to accommodate a 20ms capture with
sufficient resolution for reliable bit differentiation. Thus, in total four times
the RAM is required to add an additional channel. The PIC 18F452 only has
1.5kB of RAM, which is insufficient. Thus, the PIC 18F4620 which is pin-
compatible and has 4kB of RAM was chosen as a replacement. Unfortunately,
even operating at 32MHz, it was not fast enough to reliably capture the full
16bit timer value between every four edges on both channels simultaneously.
A further option is to clock the two CCP modules from separate timers,
however, this solution has not yet been implemented or evaluated.
5.4.6 Interface
In keeping with Texas Instruments’s Series 2000 system, RS232 was chosen
as the communication method between the RFID readers and the host system.
This ensures that should these reader devices need to be replaced at a later
date by commercial units, no hardware changes to the rest of the system will
be required.
No direct physical user interface is required, apart from a means to trigger
a read cycle (although this too could be a soft command across RS232) it was
decided to use one of the reader microprocessor’s pins as a trigger. When
this pin is pulled high, a read is triggered. Four LEDs have been added
to the reader devices to display the status (Idle, Transmitting, Receiving 1,
Receiving 2).
5.4.7 PCB Layout
The printed circuit board layout is very important as the total gain of the
final analogue receiver design is approximately 3.2 million times (65dB). This
is sufficient for received noise to be amplified into saturation.
For the reader circuit, the digital section was isolated from the analogue
receiver circuitry by using separate supply tracks and ground planes. Every
attempt was made to keep tracks as short as possible and decoupling capacitors
added wherever space permitted.
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