The CM5 (Raspberry Pi Compute Module 5) has been released. There are (and will be) countless articles about Raspberry Pi version comparisons in terms of form factor, performance, and price. However, I am going to explore the key differences between the CM4 and CM5, that we need to encounter during a custom carrier board or CM5 IO board PCB design.
Power Supply
The official CM4 IO board had 5V 3A power supply on board. And officially recommended DC adapter was also 5V 5A one. However, the new Raspberry Pi CM5 requires 5V 5A power supply for stable operation, just like the Raspberry Pi 5’s recommended DC adapter.
The official CM4 IO board used AP64501 from Diodes Semiconductor, which is rated as a 5A, synchronous buck converter with a wide input voltage range of 3.8V to 40V.
The official CM5 IO board is not released yet at the time of writing this blog post. However, with the given 5V 5A load requirement, a similar powerful bulk converter IC should be used with a larger (physically) inductor having bigger saturation current. For my first custom CM5 carrier board design, I used LMR51450 from Texas Instruments, which is rated as a 36V, 5A Synchronous Step-Down DC/DC Converter. It’s having very similar spec to AP64501 . Because I prefer to use TI’s Webbench for designing power supplies, I decided to use it.
To meet 5V 5A load requirement, the recommended inductor should have 8A saturation current. I used a 12A 4.7uH inductor from COILMX, which is bigger enough than the recommended value. As I had no strict space limitation, I could use such big one.
LMR51450 datasheet recommends to use 33uF filtering capacitors for both input and output. To achieve lower ESR with big current load, I used 4 capacitors in parallel (2 aluminum electrolytic capacitors and 2 tantalum capacitors).
USB 3.0
Raspberry Pi CM4 did not have USB 3.0 ports though it was on Raspberry Pi 4B. New Raspberry Pi CM5 added 2 USB 3.0 ports. Instead of adding a third 100pin connector like many competitors did, Raspberry Pi CM5 transformed some of existing high-speed buses into USB 3.0.
This approach of replacing some of existing DSI0 and CSI0 has its own pros and cons. However, most custom CM4 carrier boards that I had been asked to design didn’t use all 2 x DSI and 2 x CSI buses. I suspect that it was because most custom carrier boards are designed for industrial control system, where the main job was driving relays, motors, sensors, and actuators. These systems commonly had just a single touch screen interface. I haven’t seen any requirements of supporting dual touch screen among over 50 different CM4 carrier boards I’ve designed.
I didn’t use these USB 3.0 ports on my first custom CM5 carrier board design, but it would be neat to try it on my next designs.
RTC, Fan Controller, and Power Button
Because CM4 didn’t have RTC and fan controller, the official CM4 IO board had those on I2C buses. For RTC, IO board had PCF85063A from NXP Semiconductor. For fan controller, IO board had EMC2301 from Microchip Technology. And for power button, there was no easy solution for CM4, thus the official CM4 IO board didn’t have a power button switch circuit.
The Raspberry Pi CM5 now has all these on the module itself. Some reserved pins or rarely-used pins are changed for the RTC, fan controller, and power button switch.
In my first custom CM5 carrier board design, I only used the power button switch. If I needed to make the power button both compatible with both CM4 and CM5, I would probably need to add some jumer pads to connect the power button to a GPIO of CM4 or the dedicated power button pin of CM5. And using CM4’s GPIO button, it’s only possible to implement soft shutdown on OS by software, I would need to add some special logic circuit to turn on the board power on when it’s pressed (but doesn’t power off when it’s clicked again). If I have a chance to design such truly-universal carrier board compatible with both CM4 and CM5, I’ll share it here as an appendix paragraph.
Documentation
At the time of writhing this blog, the official Raspberry Pi CM5 datasheet is not released yet. Only the Forward Guidance is available via email request for downloading. All the above design changes were based on the forward guidance document. I received the document from my client, but I am sure that anyone can get access to it by signing up to the Raspberry Pi’s website. I’m too lazy of creating an account there, so I haven’t try it yet!
