Echelon FT 3150 Smart Transceiver - Manual (Page 26)

Chapter 2 - Hardware Resources
20 FT 3120 / FT 3150 Smart Transceiver Data Book
• Software command either the from the application program or from the network.
• LVI circuit detects a drop in the power supply below a set level.
During any of the reset functions, when the RESET pin is in the low state, the FT Smart Transceiver pins go to the
states described in the list below. Figure 2.11 also illustrates the condition of the pins during reset and the FT Smart
Transceivers initialization sequence after reset is returned high again.
• Oscillator continues to run
• All processor functions stop
• SERVICE pin goes to high impedance
• I/O pins go to high impedance
• All output address pins go to 0xFFFF (FT 3150 Smart Transceiver only)
• All data pins become outputs with high or low states (FT 3150 Smart Transceiver only)
• E clock goes high (FT 3150 Smart Transceiver only)
• R/W goes low (FT 3150 Smart Transceiver only)
When the RESET pin is released back to a high state, the FT Smart Transceiver begins its initialization procedure
starting at address 0x0001. The time it takes the FT Smart Transceiver to complete its initialization differs between
FT Smart Transceivers, the different firmware versions that are being run, and the memory space used by the
application (code and data). This will be discussed later in this section.
RESET Pin
The RESET pin 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:
• Watchdog Timer event.
• Software reset initialization.
• Internal LVI detects a low voltage.
• RESET pin drops below the internal trip point.
Power Up Sequence
During power up sequences, the RESET pin should be held low until the power supply is stable, to prevent start-up
malfunctioning. Likewise, when powering down, the FT Smart Transceiver RESET pin should go to a low state
before the power supply goes below the minimum operating voltage of the FT Smart Transceiver.
WARNING: If proper reset recovery circuitry is not used, the FT Smart Transceiver can go applicationless or
unconfigured. The applicationless or unconfigured state occurs when the checksum error verification routine detects
a corruption in memory which could have falsely been detected due to improper reset sequence or noise on the power
supply. Several options exist in the LonBuilder and NodeBuilder tools to allow a reboot on checksum failure.
Figure 2.11 shows typical external RESET components. The total capacitance directly connected to the RESET pin,
including stray and external device input capacitance, must not exceed 1000 pF. This ensures that the FT Smart
Transceiver can successfully output a reset down to below 0.8V. The 100 pF minimum capacitance is required for
noise immunity.