Module test board

Test board modifications

This is a list of modifications made to the test boards to correct hardware bugs. They must be implemented on new boards for them to work correctly:

Input power connector

The DC power jack (not the Mini-fit Jr. footprint) has incorrect polarity. A DC power jack connector on short flying leads with the polarity swapped should be fitted to avoid reverse polarity being accidentally applied to the board

(This has now been rectified by fitting a 2.1mm DC jack underneath the board in the MiniFit Jr. mounting holes

Addition of bulk capacitance

There is insufficient capacitance on the 3V3 line to support the voltage when the modules all power on together. This causes microcontroller brown-outs. This is rectified by fitting 2 x 470 uF 6V3 tantalum capacitors at the output of the 3V3 PSU

Replacement of electrolytic with ceramic capacitors

The input capacitors for the linear regulator (REFDES?) should be ceramic, rather than electrolytic. 1210 X5R 100 uF 6V3 capacitors fit on the same pads as the electrolytic capacitors (REFDES?) and should be fitted instead.

I2C pull-ups

1. The master I2C lines do not have pull-ups, without which they do not work. Pull-ups can be added by fitting 0603 2k2 resistors between the SCL and SDA pins and their neighbouring NC pins on the 0.1" header. The two NC pins should then be connected together and a wire connected from them to the 1V5 power supply for the TCM.
2. The pull-ups for the I2C lines for the DTMs are 10k, rather than 2k2. These should be changed

RESETB

The RESETB connections for the TCM GBTX and SCA do not account for the fact that they are both already pulled up by a 10k resistor to 1V5 and 2V5 respectively.

To fix this, the track on the test board to the 10k resistor of the potential divider should be cut. The 15k resistor of the potential divider should be fitted with a schottky diode in parallel, cathode toward uController

LED output pins

Two of the LEDs are connected to input-only pins, so will never illuminate. These are fixed by wires running from the LCD connector

Current sense ICs

The package of the IC is incompatible with the footprint. These need to be manually connected to the PCB with small flying wires or an adapter board

Clock line terminations

Some clock measurement inputs are missing 100R termination resistors. 0402 100R resistors must be fitted between the following pins (6 resistors in total):

U35: 14 & 15

U39: 1 & 2; 6 & 7; 9 & 10; 14 & 15

U42: 1 & 2

DTM RESETB signals

Voltage dividers have been added to the RESETB connections between the SCA GPIOs and the DTM RESETB inputs to reduce the 2V5 SCA outputs to 1V5. However, the voltage difference is already accounted for in the design of the DTMs and the way the GPIOs are used, so these resistors prevent the divider working.

To fix this:

The 6k8 resistors should be shorted by an 0402 0R resistor for DTM0 (R44, adjacent outer two tracks), DTM1 (R42, adjacent middle two tracks) and DTM2 (R41, adjacent middle two tracks)

The connections between the via and the 10k pull down resistors should be cut for DTM0, DTM1 and DTM2 (adjacent outer two tracks for R45, R43 and R46).

Note that all the resistor arrays must be modified rather than removed or replaced, since they are required by the TXDATAVALID signals that also use them.

ADC input decoupling capacitance

The ADC reference voltage is noisy, due to the routing of the track from the buffer and the incorrect placement of C6 at the buffer output, rather than by the ADC inputs.

This can be fixed by the addition of a decoupling capacitor of between 100 nF and 1 uF (330 nF has been tested and works well), between the ADC input signal and TCM ground, as shown in the diagram and photograph below. A 1206 capacitor is precisely the right size for this.