# Computing stuff tied to the physical world

## Anatomy of a power-up

A chip gets power, then “comes out of reset”, and that’s it, right?

Not so fast. When it comes to ultra-low power consumption, the hard part is often dominated by what the chip does before it enters its official reset state. The MPS is an excellent example – and it matters greatly because of its extremely limited energy budget.

Many problems had to be overcome to steer all the components involved into such a direction that they actually get to that reset state, and then start doing their job. Here is a scope capture of the startup process, before entering the main processing & transmit loop:

• the yellow line is Vres, the supply voltage over C1 + C2
• the red trace is calculated as the derivative (i.e. slope) of the yellow line
• the blue line is Vradio, the voltage on the RFM69’s Vcc pin

The vertical zero origin of the yellow and blue traces are just above the white “V-Marker: (CH1)” text. The derivative is centred 5 divisions from the top, and briefly goes negative. Horizontal divisions are 100 ms each, vertical is 500 mV/div for the yellow & blue traces.

This image contains all the details of the startup timeline in exquisite detail:

• the trigger point (T=0) is one division from the left edge of the graph
• power is turned on, the capacitors start to charge up
• at about T+365ms, where the dotted “1” marker is, Q1 & Q2 turn on
• Vres is 1.97V at this point, the supply to the LPC810 µC is now on
• current starts to flow, and Vres drops for the next 44 ms
• for some strange reason, explained below, Vradio starts to rise as well
• at about T+410ms, marker “2”, the µC comes out of reset and starts `main()`
• it configures various I/O pins, preventing a further rise of Vradio
• at this point, we’ve now lost 0.325V of the supply voltage, Vres
• the µC goes into power down, drawing only a few µA in this state
• now Vres can rise further, until C1+C2 have been charged up

The derivative (red line) corresponds to the rate of change of Vres, and is therefore a good indicator of the current flowing into (and briefly out of) the reservoir capacitors.