Adding a cheap and simple RTC to Rockchip devices

Background

There are plenty of nice devices these days designed around ARMv8.2 SoCs such as RK3568 or variations around RK3588. Many of them have been using the HYM8563 I2C RTC chip for about a decade. This device is reasonably cheap, requires few components, consumes very little power and is long proven to work well. Despite its low price, entry-level devices are often lacking it and only have the pads on the board, which is understandable when every dollar counts.  Some such devices include Radxa's E20C and E52C mini routers/versatile servers, which are absolutely awesome devices, which come with either dual-1Gbps or dual-2.5Gbps, feature a USB console so as to always provide a local access, and have a metal enclosure. But they're lacking the RTC, which is quite annoying for a firewall or mini-server, as a power outage always has a painful effect on the dependency chain at home (typically the OS boots faster than the ISP's box and has to start with a bad date). At least once that issue was reported, the products' creator, Tom Cubie (aka @hipboi) acknowledged the problem and suggested that new devices should have it.

Can I do it myself ?

How to proceed with existing devices in the short term ? Isn't it possible to just order the chip and solder it on board ?

One problem with HYM8563 is that it's almost always only found in TSSOP8 format, which means it's a few millimeters wide, with a pitch of 0.65mm, which means that pins are roughly 0.35mm wide with a spacing of 0.3mm between. Actually that's not that difficult to deal with, provided that you have a fine enough soldering iron. The real problem is the components that come around are also of the same scale and very close to each other, as can be seen on the photo of the E52C below (click on the photo to zoom):

The resistors and capacitors are in 0201 format, which is 0.6mm tall by 0.3mm wide, and are very difficult to solder without causing a short circuit. To get a sense of the scale above, the chip is 3mm*3mm. The crystal oscillator is a 32.768 kHz in a flat format that's not easy to find for a hobbyist.

However, the I2C pads (SDA and SCL) are "large enough" and moderately accessible, and there's power on the other side, let's keep that in mind.

Finding a pre-made board

There are various I2C RTC boards available on the net. Most of them are DS1307, and a very nice small one is based on DS3231 with a battery like this one. However, there's no HYM8563 one, and I'd prefer to stay on the same that is referenced in the DTS so that it works out of the box. But I could find the chip alone.

Making a new board instead

Then I started to think whether I could make a board myself based on the chip. Looking at the HYM8563 datasheet above reveals that it's actually not that hard to assemble the few components around. I drew a schematic on by hand (takes one minute vs one hour in Eagle):

I found a few rare occurrences of the chip in SOP8 package, which is easy to deal with, so I could make a PCB with that chip and the 6 components around it. I ordered a pack of 10pcs of that chip, and unfortunately received the tiny TSSOP8 ones instead :-(

But that reminded me that I had some small TSSOP8/SOP8/DIL PCB adapters, which support SOP8 on one side and TSSOP8 on the other side, with the DIL pads in the holes. These are convenient for on-air cabling since pads of both sides are connected together and to the holes:

After all, there were so few components that I could probably solder them directly on that PCB on either side. So let's try.

Bill of materials

Based on the schematic, I'll need this:

• 1 such PCB
  
• 1 HYM8563TS chip
• 1 diode in 0402 format
  
• 2 4.7k resistors in 0402 format
• 1 82pF capacitor
• 1 crystal oscillator
• 1 "large enough" capacitor (I counted about 6mn per 100µF), supporting at least 3.3V
  

 

The secret to avoid shorts when soldering TSSOP chips is to use solder paste. I have some in a syringe that I keep cool in the fridge (otherwise it dries in a few months and is unusable the day you need it):

Assembly steps

As a first step, we'll need to cut a trace on the PCB so as to isolate the VCC input pin from its connection to the IC's pin 8, as we'll want to place the diode there instead. We'll need to keep the resistors connected to the external VCC so that the capacitors doesn't discharge its energy into them when off. Thus we'll cut the trace after the via that connects to the other side. What's nice with these boards is that the pads of both sides correspond to the ones of the other side at the same location, so they're easy to match. The final pinout of the board will look like this:

So let's cut the trace:

Now we're going to place the chip before the diode, because it's already painful enough to solder such small chips, we don't want to be hindered by the diode. The approach for this is to place a very little drop of solder paste over all the pads on each side of the chip. Think in terms of volume, considering that the solder paste is mostly flux that will disappear (the flux will avoid shorts by making it difficult for the solder to make bridges between pins). Count that the resulting volume will be roughly 1/3 of the disposed one. Making a small trail roughly as wide as a pad is a good estimate. Make very sure not to leave any beneath the chip, as it will never melt and will stay them forever, risking to make shorts later. Only cover the pads and areas you can clean later. Once done, just place the chip over the paste:

Now solder everything with the soldering iron, without worrying about the risk of shorts, just focus on aligning the chip as best as possible with the pads, and melt absolutely all the paste. Then check with a magnifier or better, a microscope that everything's OK. With a multi-meter you need to check there's no short between adjacent pins:

Now's time to scratch the right side of the track and to place the diode, with the positive on the right and the negative on the left:

Let's now flip the board to solder the resistors. They will attach to the two bottom left pins, and connect to the beneath pad corresponding to the Vdd pin, which is in fact connected to the positive pin at the bottom right. It has the pleasant advantage of being placed immediately next to the two pins, and close enough to have the resistors directly touching on both ends:

Now let's connect the capacitor between the chip's Vss pin and the top rightmost hole that's connected to the chip's pin 1:

On the other side, the oscillator can be soldered. The pins on the left (when reading the reference) or under the notch are the useful ones. The two other ones on the right are not connected so I could connect them to the pin1 pad, though that will depend on the model since some have integrated capacitors and might not work well when doing so, but I could verify with my multi-meter that there was no capacitor there. In order to solder these pins beneath the component, solder paste is your friend again, being cautious again not to put too much:

We're only left with the capacitor that serves as an energy storage during outages. Ideally you'd use 5.5V super capacitors of 0.1F for several days, or the new 3.8V lithium-based supercaps that have even higher capacities (typically 10F and above). But given that my goal here really to just cover occasional power cuts from the mains, when a power supply dies or when I need to move cables inside my rack, I don't need more than a few minutes. And I did happen to have a 4V/150µF capacitor that perfectly matched my needs (should support 7 to 10mn of outage, and was super flat).

Soldering it is just like for the oscillator above except that it's not easy to use solder paste. The positive terminal (the one with the colored bar) needs to be connected to pin 8 of the chip (the top leftmost here) and the negative to the topmost right  hole. Soldering that one is not too hard, just melt some solder inside the hole. Alternately, using thin wires is OK as well.

Now's time to connect wires and test:

I connected these to another board that I'm using for testing I2C, and ran i2cdetect:

Good, the device appears at address 0x51, so at least it's detected. Now we need to verify that its oscillator is properly ticking. For this we'll have to write the current time to the chip (which appears as rtc1 on this board). It will emit an "invalid argument" because it first tries to read the time which contains a bit "VL" ("Voltage Low") set in the second field indicating the the device lost power since last time it was set. But we can ignore it when writing the date, reading it next will indicate whether it works or not (the time must change):

Perfect, it works! Now's time to replace my testing wires with thin ones, to put all of that into shrink tube to protect it and solder it in the device.

Installation

Let's spot the 4 pads we'll need inside the E52C. First, at the bottom of the board, we'll find the I2C SDA and SCL pins at the bottom on these photos, where we'll solder our SDA/SCL wires (violet and green here). They'll have to pass between the board and the enclosure so they must be very thin, but where I'm passing them, there's enough room:

Next step is to install the module on the other side of the board. We're going to glue it on top of the micro SD card reader with some double-sided tape. The capacitor close to it has both positive (3.3V) and negative and is large enough to support soldering directly to it:

Conclusion

It was fun to make but took me most of the day to build; soldering small components requires delicate manipulations, and dealing with flux and solder paste requires lots of cleaning along the operations. I've made two modules so I still have one extra left. This will allow me to migrate another of my machines to a new one, but I'm impatient to see them produced with the chip already soldered so that I don't have to do this anymore!