Raspberry Pi 4 Compute simplifies power pinout
“Compute Module 4 introduces a brand new form factor, and a compatibility break with earlier Compute Modules,” said Ebon Upton, founder of the Raspberry Pi Foundation and SoC architect at Broadcom in Cambridge. “Where previous modules adopted the JEDEC DDR2 SODIMM mechanical standard, with I/O signals on an edge connector, we now bring I/O signals to two high-density perpendicular connectors (one for power and low-speed interfaces, and one for high-speed interfaces),” . “This significantly reduces the overall footprint of the module on its carrier board.”
“Over half of the seven million Raspberry Pi units we sell each year go into industrial and commercial applications, from digital signage to thin clients to process automation,” he said. “Many of these applications use the familiar single-board Raspberry Pi, but for users who want a more compact or custom form factor, or on-board eMMC storage, Compute Module products provide a simple way to move from a Raspberry Pi-based prototype to volume production,” said Upton
“Previous Compute Modules were all in a 200-pin SODIMM form factor, but two important considerations pushed us to think about moving to a different form factor: the need to expose useful interfaces of the BCM2711 that are not present in earlier SoCs, and the desire to add extra components, which meant we needed to route tracks differently to make space on the PCB for the additional parts,” said the designer Dominic Plunkett.
“Whilst it was easy to add some headline features like a second HDMI port, other useful features don’t grab as much attention,” he said. “One example is that we have simplified the powering requirements. Previous Compute Modules required multiple PSUs to power a board, and the power-up sequence had to be exactly correct. Compute Module 4 simply requires a single +5V PSU.”
“In fact, the simplest possible base board for Compute Module 4 just requires a +5V supply and one of the connectors and nothing else,” he said. “You would need a CM4 variant with eMMC and wireless connectivity; you can boot the module with the eMMC, wireless connectivity gives you networking, and Bluetooth connectivity gives you access to IO devices. If you do add extra IO devices the CM4 also can provide a +3.3V supply to power those devices, avoiding the need for an external power supply.”
Next: High speed signals on Raspberry Pi 4 Compute board
The designers also wanted to add extra items to the PCB, so PCB space to put the additional parts was an important consideration. The previous Compute Module 3 has tracks carrying signals from one side of the SoC to the pins on the edge connector. These tracks take up valuable PCB space, preventing components being fitted there. We could add extra PCB layers to move these tracks from an outer layer to an inner layer, but these extra layers add to the cost of the product.
“We knew we wanted to get the extra features of the BCM2711 out to the connector so that users could make use of them in their products,” said Plunkett. “High-speed interfaces like PCIe and HDMI are so fast coming out of the BCM2711 that they need special IO pins that can’t also support GPIO: if we were to change the functionality of a GPIO pin to one of the new high-speed signals, this would break backwards compatibility.
The board is based on the same 1.5GHz 64-bit quad-core BCM2711 application processor as Raspberry Pi 4 to provide more performance for embedded designs.
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