How to prepare the UART cable’s wires (Raspberry Pi 5)¶
You have the parts for an Acorn on a Raspberry Pi 5 and are making the second of its two cables, from the Acorn’s P2 socket (the serial port). On this page: find wire 1, check it with a meter, cut back the wires that are not used, crimp the rest.
This procedure is waiting for its bench run: issue #218.
Photos: Waveshare (PoE M.2 HAT+ (B)), RHS Research (LiteFury underside; the Acorn is the same PCB).
What you need¶
Parts
the other half of the Molex Pico-EZmate cable, which was cut in half in How to prepare the JTAG cable’s wires (Raspberry Pi 5)
the 2×3 Dupont housing
5 Dupont crimp terminals, and a few spare
2 mm heat-shrink tube
the Acorn
Tools
a multimeter with a continuity buzzer, and a fine probe tip for it or a sewing pin to hold against a probe
a ruler marked in millimetres
side cutters, wire strippers and the crimping tool
a hot-air tool
masking tape and a fine pen
a screwdriver for the HAT’s M.2 mounting screw
Steps¶
1. Have the Acorn out of its slot. If it is fitted, the Raspberry Pi 5 must be off first. Power off the Raspberry Pi 5: unplug its power, and its network cable if it is powered over PoE.
Then take the card out. Take the screw at its far end out first: the SSD mounting screw the HAT ships with (Waveshare’s page for the HAT).
The Acorn is a bare circuit board. Before you pick it up, touch bare metal of the host, or wear an anti-static wrist strap; hold the card by its edges.
If the Molex cable is still whole, do the reach check first: step 2 of How to prepare the JTAG cable’s wires (Raspberry Pi 5). Then cut it in the middle with side cutters: each half is one cable.
2. Find wire 1 of the P2 cable and flag the wires, before cutting any wire back.
Press the plug of the other half into socket P2 on the underside of the Acorn. Hold the card underside up with the M.2 edge to your left: wire 1 is the leftmost. In the picture under this step, the card in the photograph has a cable already in P1 and nothing in P2. Your card has nothing in either socket until you press a plug in.
Put a numbered tape flag on each of the 6 wires, 1 at the left to 6, about 25 mm from the cut end. Keep each flag clear of the end that will be cut and stripped later. Write P2 on the flag of wire 1 as well: off the card, the two halves look alike.
3. Check which wire of the P2 cable is wire 1, with a meter, before cutting any wire back.
With the plug still in the socket (the Acorn out of any slot), set the meter to continuity. Press the probe tip, or a pin held to it, against the cut face of the wire flagged 1 (it is not stripped yet). Put the other probe on the plated half-round mounting pad at the end of the card, not on the round holes. It must beep.
Do the same with the wire flagged 6: it must stay silent.
If wire 6 beeps instead, stop: the numbering is reversed; take the flags off and number from the other end. Then do items 1 and 2 of this step again, with the flags renumbered. If neither wire beeps, strip about 3 mm from wires 1 and 6 and try again. If still neither beeps, stop: the ground point is not confirmed.
If both wires beep, first see that the two probes do not touch each other. See also that the cut faces of wires 1 and 6 do not touch. If both still beep, stop and cut nothing: the check cannot tell the wires apart. Leave the flags on and take the plug out. Wire 6 is the card’s 3.3 V by the LiteX wiki’s legend. It is expected to stay silent to ground on a card with no power. Set the meter to ohms. Write down what each of the two wires reads to the pad. Send both readings to the fpgas.online project in an issue at fpgas-online/fpgas.online-test-designs.
Take the plug out again.
4. Cut wire 6 off about 10 mm from the plug. Shrink a piece of the 2 mm tube over the cut end. It goes in no cavity. Wire 6 is VCC, 3.3 V from the Acorn by the LiteX wiki’s legend: it must never reach the host. Leave wires 1 to 5 at full length.
5. Strip about 3 mm from wires 1 to 5. Crimp a Dupont terminal on each.
Check¶
Wire 1 of the P2 cable is the one the meter check of step 3 found. Each wire that goes in the housing carries its numbered flag and has a terminal crimped on.
If it fails¶
The meter check of step 3 finds the numbering reversed, or finds both wires beeping: that step says what to do, and when to stop.