Lab 2
Board anatomy and power rails
Find every major part on your board and in its layout, trace the 5 V input to every rail, then measure the rails and the current the whole board draws at idle and under load.
Modules this lab uses
What you need
- Your station's board, power supply and boot media from lab 1
- An inline USB Type-C power meter (voltage and current), from your station
- The bench multimeter
- The board's schematic and its layout and power viewers, linked from the modules
Before you start
Read the rules that keep the board alive. Probe carefully: a multimeter probe that slips between two pins can short a rail. Put the power meter in line with the board's supply only while the board is unplugged.
Steps: the parts
- Find the system-on-chip, the DRAM, the SPI NOR flash, the regulators, the crystal and the connectors on the board in front of you. Photograph the board and label each one.
- Find the same parts in the board layout viewer, and follow one trace from the system-on-chip to a connector.
- Open the schematic and fill in a table: part, function, and what the board would do at power-on without it.
Steps: the rails
- Trace the 5 V input to every rail in the power viewer and the schematic: which regulator makes it, and what that regulator's enable depends on. Module 2 lists Das Frite's rails with each one's maximum current and the power-on order.
- Measure each rail with the multimeter at the test points your TA has marked for your board, and compare each with its nominal value.
- With the board unplugged, put the USB power meter between the supply and the board. Power on and record the board's current at idle, then under a CPU load that keeps every core busy: start one busy loop per core with
for i in $(seq $(nproc)); do yes > /dev/null & done, and stop them afterwards withkillall yes. - Read the supplied oscilloscope traces of power-up, and mark the order the rails rise and the moment reset is released.
Steps: the rails
- Trace the 5 V input to every rail in the power viewer and the schematic: which regulator makes it, and what that regulator's enable depends on. The viewer shows the enable and power-good signals on each connection.
- Measure each rail with the multimeter at the test points your TA has marked for your board, and compare each with its nominal value.
- With the board unplugged, put the USB power meter between the supply and the board. Power on and record the board's current at idle, then under a CPU load that keeps every core busy: start one busy loop per core with
for i in $(seq $(nproc)); do yes > /dev/null & done, and stop them afterwards withkillall yes. - Read the supplied oscilloscope traces of power-up, and mark the order the rails rise and the moment reset is released.
What to measure
- Each rail's voltage, beside its nominal value.
- The board's current and power at idle and under load, compared with the board's rated figures in module 2.
- The order the rails rise on the supplied traces, and when reset is released.
What you hand in
- A labelled board photo.
- A rail table: rail name, nominal voltage, measured voltage, and the regulator that feeds it.
- A current chart for idle and load, with its axis starting at zero and the unit in the axis title.
- One paragraph on why the rails come up in sequence before reset is released.
- Your own short report: what you did, what you measured, and one problem you met and how you solved it.
If it goes wrong
If the board stops booting after this lab, work down when it will not boot on your bench page, and tell your TA if a probe slipped.