User guide

FT 3120 / FT 3150 Smart Transceiver Data Book 67
Serial I/O Objects
The duration of this function call is a function of the number of data bits transferred and the transmission bit rate. t
fin
(max) refers to the maximum amount of time this function will wait for a start bit to appear at the input. After
this time, the function will return a 0 as data. t
fin
(min) is the time to the first sampling of the input pin. As an
example, the timeout period at 2400 bits/second is (20 x 10 x 1/2400) + t
fin
(min).
Figure 3.32 Serial Output
The duration of this function call is a function of the number of data bits transferred and the transmission bit rate. As
an example, to output 100 bytes at 300 bits/second would require a time duration of (100 x 10 x 1/300) + t
fout
+ t
ret
.
Touch Input/Output
The Touch I/O object enables easy interface to any slave device which adheres to the Dallas Semiconductor Touch
Memory™ standard. This interface is a one-wire, open-drain, bidirectional connection.
Up to eight Touch Memory busses may be connected to a FT Smart Transceiver through the use of the first eight I/O
pins, IO0 – IO7. The only additional component required for this is a pull-up resistor on the data line (refer to the
Touch Memory specification below on how to select the value of the pull-up resistor). The high current sink
capabilities of IO0 – IO3 pins of the FT Smart Transceiver can be used in applications where long wire lengths are
required between the Touch Memory device and the FT Smart Transceiver.
The slave acquires all necessary power for its operation from the data line. Upon physical connection of a Touch
Memory device to a master (in this case the FT Smart Transceiver), the Touch Memory generates a low presence
pulse to inform the master that it is awaiting a command. The FT Smart Transceiver can also request a presence pulse
by sending a reset pulse to the Touch Memory device.
Commands and data are sent bit by bit to make bytes, starting with the LSB. The synchronization between the FT
Smart Transceiver and the Touch Memory devices is accomplished through a negative-going pulse generated by the
FT Smart Transceiver.
Figure 3.33 shows the details of the reset pulse in addition to the read/write bit slots.
Symbol Description Typ @ 10MHz
t
fout
Function call to start bit 79 µs
t
ret
Return from function 10 µs
END OF
FRAME
DATA
ONE FRAME
START BIT
APPEARS
START OF
io_out()
t
fout
87654321
TIME
SERIAL
OUTPUT
START START STOP
END OF
io_in()
t
ret