Echelon I/O Model Reference for Smart Transceivers and Neuron Chips - Manual (Page 130)

120 Serial I/O Models
• The zero data bit signal
• A one data bit signal
Data pulses appear exclusively of each other and are typically spaced
approximately 1 ms apart. Specifications for the duration of the pulse are
typically between 50 to 100 μs, but they can be as short as 50 ns for a Series 3100
device with a 40 MHz input clock, or 12.5 ns for a Series 5000 device with an 80
MHz system clock. Table 48 shows the pulse width and inter-bit period (the
period between bit pulses).
Table 48. Wiegand Pulse Width and Inter-Bit Period
Parameter Minimum Maximum Typical
Pulse Width 200 ns 880 ms 100 μs
Inter-Bit Time 150 μs None 900 μs
Wiegand data is asynchronous. The io_in( ) function must be executing before
the second bit arrives, otherwise the first bit data is lost because it then becomes
impossible to determine the order of a zero and one event sequence. Data is read
most-significant bit (MSB) first, that is, the first data bit read will be stored in
the most significant bit location of the first byte of the array when eight bits are
read into that byte. If the number of bits transferred is not a multiple of eight, as
defined by count , the last byte transferred into the array contains the remaining
bits right justified within the byte.
For Wiegand input, one of the IO_0 through IO_7 pins can optionally be
designated as a timeout pin. A logic one level on the timeout pin causes the
Wiegand input operation to terminate before the specified number of bits has
been transferred. The Neuron Chip or Smart Transceiver updates the watchdog
timer while waiting for the next zero or one data bit to arrive. This timeout input
can be a one-shot timer counter output, an RC circuit, or a ~Data_valid signal
from the reader device.
The return_value for the io_in( ) function for this model is an unsigned short, and
it indicates the number of bits stored into the array. Whenever the io_in( )
function for a Wiegand I/O object is called, it immediately returns if there is
currently no activity on the indicated I/O pins. Otherwise, the function continues
to process input data until either
count bits are stored, or until the timeout event
occurs. When the timeout event occurs, the number of bits read and stored is
returned. The io_in( ) function is blocking, and can take more than one second to
process the card information, depending upon the speed at which the card travels
through the reader. Because this function ties up the application processor, it
handles updates to the watchdog timer.
Syntax
pin [input] wiegand [timeout( pin-nbr )] io-object-name ;
pin
An I/O pin. Wiegand input requires two adjacent pins. The DATA 0 pin is
the pin specified, and the DATA 1 pin is the following pin. The pin