Echelon FTXL Hardware - Manual (Page 33)

FTXL Hardware Guide 25
Transceiver, this pin is not part of the communications port, and does not
function as a sleep control pin, but acts as in interrupt control pin.
The FTXL LonTalk protocol stack uses the IRQ pin to receive an indication from
the FTXL Transceiver that the network is ready, either for uplink or for
downlink. The FTXL LonTalk protocol stack asserts the IRQ pin high to cause
the interrupt.
The
downlink ready interrupt allows the FTXL Transceiver to inform the host
when it has read the first byte of the transfer. This interrupt accounts for the
latency of the parallel interface, that is, between a host write for a downlink
transfer and the FTXL Transceiver read for the transfer. This latency is on the
order of 110 microseconds (for an FTXL Transceiver running at 20 MHz), but it
could be longer if the FTXL Transceiver is busy processing an incoming network
frame. The FTXL LonTalk protocol stack initiates a downlink transfer by writing
only the length byte; it then lets the interrupt service routine handle the rest of
the transfer.
The
uplink ready interrupt is an indication from the FTXL Transceiver that
uplink traffic needs to be transferred. The IRQ pin is asserted only when the
FTXL Transceiver does not own the write token.
The IRQ pin is deasserted during the downlink activity.
Although there are two interrupt cases, there is only a single interrupt request
line. The interrupt type is determined by the FTXL LonTalk protocol stack based
on the state of the FTXL Transceiver and token ownership.
Reset Pin
The RESET~ pin (pin 40) is both an input and an output. As an input, the
RESET~ pin is internally pulled high by a current source acting as a pull-up
resistor. The RESET~ pin becomes an output when any of the following events
occur:
• Software reset
• Watchdog Timer event
• Internal low-voltage inhibit (LVI) circuit detects a low voltage
• RESET~ pin drops below the internal trip point
Important: The Nios II processor must be able to detect changes to and assert
the FTXL Transceiver’s RESET~ pin.
Reset Function
The reset function is a critical operation for any embedded microcontroller. In
the case of an FTXL Transceiver, the reset function plays a key role in the
following conditions:
• Initial V
DD
power up
Reset ensures proper initialization of the FTXL Transceiver.
• V
DD
power fluctuations
Reset manages proper recovery of the FTXL Transceiver after V
DD
stabilizes.