Module 2 · Lab 2
First power-on and voltage rails
Before the first instruction runs, the board has to turn one 5 V input into several lower voltages, bring them up in the right order, start its clock and let go of reset. You trace this on your own board and measure it.
Key ideas
- One input, many rails. Both course boards take 5 V over USB Type-C. The system-on-chip's CPU cores, the rest of its logic, its I/O and the memory each need their own supply, called a rail.
- Two kinds of regulator. A buck converter switches to step voltage down efficiently, for rails that carry a lot of current. A linear regulator (LDO) is simpler and quieter, for small loads.
- Adjustable rails. The CPU rail can be changed at run time, so the OS can lower the voltage when it lowers the clock and save power.
- Sequencing. Rails must rise in a set order, and the chip is held in reset until they are stable. A reset supervisor or a power-good signal decides when reset is released; then the crystal is running and the CPU fetches its first instruction from the boot ROM.
- The header is 3.3 V. A 5 V signal on a GPIO pin can damage the system-on-chip. The pinout colours supply pins by voltage so this is hard to miss.
Das Frite's power tree
Das Frite's rails
From the board's schematic. The input is 5 V at up to 2 A over USB Type-C. The power viewer draws the same tree from the board's own design data.
| Rail | Made by | Feeds | Maximum |
|---|---|---|---|
| VDD_CORE (VCCK) | Buck converter, voltage set by a PWM signal from the chip | The CPU cores and GPU | 3 A |
| VDD_EE | Buck converter, voltage set by a PWM signal from the chip | The rest of the system-on-chip's logic | — |
| VDDQ 1.2 V | Buck converter | The DDR4 memory and the chip's memory interface | 2 A |
| VDDIO_AO 3.3 V / VCC 3.3 V | Buck converter | I/O, the 40-pin header's 3.3 V pins | 2 A |
| VCC 1.8 V | Linear regulator from 3.3 V | 1.8 V I/O | 500 mA |
| DDR4 2.5 V | Linear regulator from 3.3 V | The DDR4 chip's word-line supply | 500 mA |
| VCC5V | A switch from the 5 V input | HDMI, USB and the audio amplifier | 1 A |
Power-on order on the schematic: 1. VDD_CORE, 2. 3.3 V, 3. 1.8 V and DDR4 2.5 V, 4. VDDQ 1.2 V. A reset supervisor holds the chip in reset until the supply is good. La Frite (AML-S805X-AC), the first version of this board, has the same power circuits and is rated under 0.75 W idle, 2.5 W typical and 4 W at full load.
Solitude's rails
The input is 5 V at up to 3 A over USB Type-C. The power viewer draws every regulator and the rails it feeds, with the enable and power-good signals on each connection, read from the board's own design data. The product page rates Solitude under 1.5 W idle, 3 W typical and 5 W at full load.
On the bench
In short, what you do in lab 2, where the steps are given in full:
- Trace the input to every rail: which regulator makes it, and what that regulator's enable depends on.
- Measure each rail at the test points your TA marks for your board and compare with its nominal value.
- With an inline USB power meter, measure the board's current at idle and under a CPU load, plot both on an axis that starts at zero, and compare with the rated figures.
- Read the supplied oscilloscope traces of power-up and mark the order the rails rise and the moment reset is released.
Resources
| Resource | Use in this module |
|---|---|
| Das Frite power | Which regulator feeds which rail, with enable and power-good signals on each connection |
| Das Frite board layout | Where each regulator sits; right-click a pad to follow its net |
| Board hardware schematics links | The power pages of the schematic |
| La Frite product page | Power figures for the same power circuits |
Resources
| Resource | Use in this module |
|---|---|
| Solitude power | Which regulator feeds which rail, with enable and power-good signals on each connection |
| Solitude board layout | Where each regulator sits |
| Board hardware schematics links | The power pages of the schematic |
| Solitude product page | Input rating and power figures |