7. Functional Description
This section documents the functions and behavior of the RC switch. The information in this section is not needed to use the device, but it might be helpful if you are curious about the details.
The timing accuracy of the RC switch is limited by its internal clock, which has an accuracy of ±2%. Because of this, the threshold and other timing parameters might vary by a few percent.
The device stores two user-settable configuration parameters: threshold and inversion.
The threshold parameter is a pulse width that can range from 900 to 2100 μs and must be a multiple of 16 μs. The default threshold is 1696 μs.
The inversion parameter can either be enabled or disabled. When inversion is disabled, the pulses have to be above the threshold in order to activate the switch. When inversion is enabled, the pulses have to be below the threshold in order to activate the switch. By default, inversion is disabled.
The device measures standard RC pulses on its RC input (which is labeled RC IN or SEL depending on the device). It is designed to have strict requirements for the signal and to deactivate itself as soon as possible when those requirements are not met. The device expects the pulses to have a width within 500 to 2500 μs and a frequency within 10 to 333 Hz. These limits cannot be strictly enforced because of the ±2% clock accuracy, but the device is designed to accept any signal that is within these limits. If there has been no activity for 130 ms, the signal is considered to be bad. If there have been too many glitches detected on the RC input, the signal is considered to be bad.
The device requires three good pulses in a row before it will consider the signal to be good. However, as soon as the signal violates any of the rules above, it will consider the signal to be bad and deactivate the switch.
When comparing the measured pulse width to the threshold to determine whether the pulses are above or below the threshold, the device uses a hysteresis of ±64μs.
All of the RC switches except the Pololu 4-Channel RC Servo Multiplexer have a safe-start feature that reduces the likelihood of unexpected activation. Safe-start mode is active at power up, and while the safe-start mode is active, the switch does not activate. The device deactivates safe-start mode when the signal is good and it receives a pulse in the off position. The device will re-activate safe-start mode if the signal is bad continuously for 500 ms.
If the device is receiving a valid RC signal which suddenly stops, it will take 130 ms before the signal is recognized as bad. From there, it will take 500 ms before the safe-start mode becomes active, so there will be a total of 630 ms between the loss of the signal and the re-activation of safe-start mode.
To get out of safe-start mode and activate the switch, you should move the input to the off position and then move it to the on position.
Even with the safe-start feature, unexpected activation of the switch is still possible. With the Pololu RC Switch with Relay, one way this could happen is if the switch is activated but the logic power supply is at 3 V, which is too low to actually turn on the relay. If the logic power rises from 3 V to 5 V then the relay could turn on unexpectedly.