Production and testing: Fault analysis

Component placement and orientation

Supply voltages

Clock frequency

The flawless performance of the crystal oscillator is crucial for operation. Anomalies are easy to detect with even a low-cost oscilloscope and pinpoint specific problems and help to select further analysis steps.

The crystal used in atben and atusb has a nominal tolerance of +/- 15 ppm at 22-28 C. Low-cost oscilloscopes typically have a timing accuracy of +/- 100 ppm, which means that only major excursions can be detected by measuring the clock output with such an instrument. Full-speed USB only requires an accuracy of +/- 2500 ppm. We can therefore consider all results within a range of +/- 1000 ppm as sufficient, and perform more precise measurements by other means. This applies to atben as well as to atusb.

Measuring the clock on atben

atben normally does not output a clock signal. A 1 MHz clock can be enabled with the following command:

atrf-txrx -d net:ben -C 1

This configures atben as a promiscuous receiver. The reception of any IEEE 802.15.4 frame or pressing Ctrl-C will terminate the command.

Clock Action
0 Hz Check voltages; check that the clock is enabled; check for shorts around crystal; check connectivity of crystal
0.999-1.001 MHz, ~3.3 Vpp Perform precision measurement with atrf-xtal
Other Check voltages; check for contamination around crystal

Clock	Action
0 Hz	Check voltages; check that the clock is enabled; check for shorts around crystal; check connectivity of crystal
0.999-1.001 MHz, ~3.3 Vpp	Perform precision measurement with atrf-xtal
Other	Check voltages; check for contamination around crystal

Measuring the clock on atusb

The transceiver provides the clock for the microcontroller in atusb. A clock signal is therefore always available. Immediately after reset, the transceiver generates a 1 MHz clock. When the microcontrolled comes out of reset, it raises the transceiver's clock output to 8 MHz and then enables USB.

Clock Action
0 Hz Check voltages; check for shorts around crystal; check connectivity of crystal
0.999-1.001 MHz, ~3.3 Vpp Check presence of firmware; check for shorts on SPI signals; check connectivity of SPI signals
7.992-8.008 MHz, ~3.3 Vpp Perform precision measurement with atrf-xtal
Other Check voltages; check for contamination around crystal

Clock	Action
0 Hz	Check voltages; check for shorts around crystal; check connectivity of crystal
0.999-1.001 MHz, ~3.3 Vpp	Check presence of firmware; check for shorts on SPI signals; check connectivity of SPI signals
7.992-8.008 MHz, ~3.3 Vpp	Perform precision measurement with atrf-xtal
Other	Check voltages; check for contamination around crystal

Precision measurements

The clock frequency of atben can be measured with an accuracy of about +/- 100 ppm using the program atrf-xtal. atrf-xtal runs directly on the Ben and measures the duration of packet transmissions. The transmission time depends on the bit clock which is in turn derived from the oscillator.

atrf-xtal requires exclusive access to the atben. If a proxy is running, it has to be removed first.

killall atrf-proxy
atrf-xtal 100

The number reported is the number of poll loops the CPU counted. This value should be compared to a reference count obtained with a known to be good atben board and the same Ben at a comparable temperature.

Difference Action
> +/- 50 ppm Correct operation
< -80 ppm Check soldering of capacitors; check for contamination around crystal
> +120 ppm idem
Other Divergence can be compensated by adjusting trim value

Difference	Action
> +/- 50 ppm	Correct operation
< -80 ppm	Check soldering of capacitors; check for contamination around crystal
> +120 ppm	idem
Other	Divergence can be compensated by adjusting trim value

Up: Production and testing Prev: Functional test

Last update: 2011-05-18 Werner Almesberger