A power-gating timer that switches its load off too soon
Symptom
Section titled “Symptom”A load whose supply a nano-power timer switches (the timer turns a power switch on at intervals, and off again when the load signals that it has finished) switches off right after it starts, runs only once, or behaves differently from board to board.
Two inputs of such a timer, for example the TPL5110 [Texas Instruments, TPL5110 Nano-Power System Timer for Power Gating], are easy to leave undefined:
- Its EN/ONE_SHOT pin selects between periodic operation and a single cycle, and it must be tied to GND or VDD with short traces [Texas Instruments, TPL5110 Nano-Power System Timer for Power Gating]. A floating pin’s level is undefined, and so is the mode.
- Its DONE input is the load’s signal that it has finished. The timer takes a low-to-high transition as a valid DONE and switches the load off, and a pulse as short as 100 ns is enough [Texas Instruments, TPL5110 Nano-Power System Timer for Power Gating]. A stray rising edge that lasts that long ends the cycle just as a real DONE would: from a controller’s pin, for example, while the controller starts up and before its firmware drives that pin.
How to confirm
Section titled “How to confirm”Watch DONE with an oscilloscope while the load powers up, triggered on a rising edge, with a timebase short enough to show a pulse of the data sheet’s minimum width. A pulse there before the firmware signals that it has finished is the cause. Check that the mode pin connects straight to GND or VDD.
Tie the mode pin to GND or VDD with a short trace [Texas Instruments, TPL5110 Nano-Power System Timer for Power Gating]. Since a rising edge on DONE ends the cycle [Texas Instruments, TPL5110 Nano-Power System Timer for Power Gating], hold DONE low with a pull-down resistor, so that it stays low while nothing drives it, and have the firmware raise it only when the work is done. Then repeat the check above.
References
Section titled “References”- [Texas Instruments, TPL5110 Nano-Power System Timer for Power Gating]: the mode pin, tied to GND or VDD, and the DONE input: a low-to-high transition, and the shortest valid pulse.
The numbers in the examples, presets and exercises on this site are synthetic: round teaching values, not data from any real design.