Raspberry Pi Compute Module 4

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Chapter 1. Introduction

1.1. Introduction

The Raspberry Pi Compute Module 4 (CM4) is a System on Module (SoM) that integrates a processor, memory, eMMC Flash, and power circuitry. It enables designers to incorporate Raspberry Pi hardware and software into their custom systems and form factors. The CM4 offers additional I/O interfaces beyond those found on standard Raspberry Pi boards, providing greater design flexibility.

The CM4 design is based on the Raspberry Pi 4 Model B. For cost-sensitive applications, a version without eMMC, known as the Raspberry Pi Compute Module 4 Lite (CM4Lite), is available.

Unlike previous Compute Modules that shared a DDR2-SODIMM mechanical form factor, the CM4 and CM4Lite feature a new design. Their electrical interface uses two 100-pin high-density connectors, and the physical form factor is more compact when connectors are considered.

This redesign accommodates new interfaces such as a second HDMI port, PCIe, and Ethernet, which were not feasible with the previous form factor.

Note: Unless otherwise stated, "CM4" in this document also refers to CM4Lite.

1.2. Features

• Broadcom BCM2711, quad-core Cortex-A72 (ARM v8) 64-bit SoC @ 1.5GHz
• Small Footprint: 55mm x 40mm x 4.7mm module
• 4 x M2.5 mounting holes
• H.265 (HEVC) up to 4Kp60 decode, H.264 up to 1080p60 decode, 1080p30 encode
• OpenGL ES 3.0 graphics
• Options for 1GB, 2GB, 4GB, or 8GB LPDDR4-3200 SDRAM with ECC (see Appendix B)
• Options for 0GB (CM4Lite), 8GB, 16GB, or 32GB eMMC flash memory (see Appendix B)
• Peak eMMC bandwidth of 100MBps (four times faster than previous Compute Modules)
• Optional certified radio module (see Appendix B) with:
  • 2.4 GHz, 5.0 GHz IEEE 802.11 b/g/n/ac wireless
  • Bluetooth 5.0, BLE
  • On-board electronic switch for selecting between PCB trace or external antenna
• Gigabit Ethernet PHY supporting IEEE 1588
• 1 x PCIe 1-lane Host, Gen 2 (5Gbps)
• 1 x USB 2.0 port (high speed)
• 28 x GPIO pins supporting 1.8V or 3.3V signalling and peripheral options:
  • Up to 5 x UART
  • Up to 5 x I2C
  • Up to 5 x SPI
  • 1 x SDIO interface
  • 1 x DPI (parallel RGB display)
  • 1 x PCM
  • Up to 2 x PWM channels
  • Up to 3 x GPCLK outputs
• 2 x HDMI 2.0 ports (up to 4Kp60 supported)
• MIPI DSI:
  • 1 x 2-lane MIPI DSI display port
  • 1 x 4-lane MIPI DSI display port
• MIPI CSI-2:
  • 1 x 2-lane MIPI CSI camera port
  • 1 x 4-lane MIPI CSI camera port
• 1 x SDIO 2.0 (CM4Lite)
• Single +5V PSU input.

Chapter 2. Interfaces

2.1. Wireless

The CM4 can be equipped with an on-board wireless module based on the Cypress CYW43455, supporting both 2.4 GHz and 5.0 GHz IEEE 802.11 b/g/n/ac wireless, and Bluetooth 5.0, BLE. These interfaces can be enabled or disabled as needed. For example, in a kiosk application, a service engineer could enable wireless operation and then disable it.

The CM4 features an on-board antenna. If used, it should be positioned away from metal, including ground planes (refer to Chapter 3). An alternative standard U.FL connector is available for an external antenna. Raspberry Pi Ltd provides an antenna kit certified for the CM4. Using a different antenna requires separate certification.

Warning: Raspberry Pi Ltd cannot assist with certification for third-party antennas.

Antenna selection (internal or external) is configured via the config.txt file at boot time. Use dtparam=ant1 for the internal antenna or dtparam=ant2 for the external antenna.

2.1.1. WL_nDisable

This pin has multiple functions:

1. Monitors the wireless networking module's enable/disable state (high indicates powered up).
2. When driven low, it prevents the wireless module from powering up, saving power or ensuring the interface is disabled. Re-initialization of the driver is needed if the interface is enabled after being disabled.

Note: On CM4 modules without wireless, this pin is reserved.

2.1.2. BT_nDisable

This pin also has multiple functions:

1. Monitors the Bluetooth module's enable/disable state (high indicates powered up).
2. When driven low, it prevents the Bluetooth module from powering up, saving power or ensuring the interface is disabled. Re-initialization of the driver is needed if the interface is enabled after being disabled.

Note: On CM4 modules without wireless, this pin is reserved.

2.2. Ethernet

The CM4 includes an on-board Gigabit Ethernet PHY, the Broadcom BCM54210PE, featuring IEEE 1588-2008 compliance and MDI crossover, pair skew, and pair polarity correction. A standard 1:1 RJ45 MagJack provides the Ethernet connection. Figure 2 illustrates typical wiring for a MagJack with PoE and ESD protection.

Differential Ethernet signals should be routed as 100Ω differential pairs with appropriate clearances. Length matching between pairs should be better than 50mm, and within a pair, ideally better than 0.15mm.

The PHY supports up to three low-active LEDs for status feedback, configurable by the OS driver. It also provides SYNC_IN and SYNC_OUT signals at 3.3V for IEEE 1588-2008 support.

2.3. PCIe (Gen2 x1)

The CM4 integrates a PCIe 2.0 x1 host controller. Unlike the Raspberry Pi 4 Model B, where this controller is connected to a USB 3 host controller, the CM4 allows designers to choose its interface usage.

Warning: Ensure a suitable OS driver is available for the chosen host controller before prototyping.

Note: The on-board PCIe Host controller does not support 64-bit accesses from the ARM; these must be split into two 32-bit accesses.

Connecting a PCIe device follows standard PCIe conventions. The CM4 includes on-board AC coupling capacitors for CLK and PCIe_TX signals. PCIe_RX signals require external coupling capacitors near the driving source. PCIe conventions dictate swapping TX/RX pairs when wiring directly to an IC. The PCIe_CLK_nREQ must be connected for clock signal generation, and PCIe_nRST for device reset.

Differential PCIe signals should be routed as 90Ω differential pairs with suitable clearances. While pair-to-pair length matching is not critical, signals within a pair should be length-matched to better than 0.1mm.

Tip: Kernels 5.10 and newer support MSI-X, with up to 32 IRQs available. If interrupt issues occur, adding pci=nomsi to cmdline.txt may resolve them.

2.4. USB 2.0 (high speed)

The USB 2.0 interface supports up to 480Mbps signaling. Differential pairs should be routed as 90Ω pairs, with signal length matching ideally better than 0.15mm.

Tip: The firmware disables the USB interface by default for power saving. In recent Raspberry Pi OS (Bullseye) versions, it is automatically enabled with otg_mode=1 in config.txt. For other OS versions or older Raspberry Pi OS, add this setting to config.txt.

Note: The port can function as a USB On-The-Go (OTG) port. Users have reported success with this functionality. The USB_OTG_ID pin selects between USB host and device modes. For fixed slave or master operation, tie the USB_OTG_ID pin to ground.

2.5. GPIO

There are 28 General Purpose I/O (GPIO) pins, corresponding to the Raspberry Pi 4 Model B's 40-pin header. These pins provide access to internal peripherals like SMI, DPI, I2C, PWM, SPI, and UART. The BCM2711 ARM peripherals book details these features and multiplexing options. Drive strength and slew rate should be minimized to reduce EMC issues. GPIO2 and GPIO3 have 1.8kΩ pull-up resistors.

The BCM2711 GPIO bank is powered by GPIO_VREF, which can be connected to +1.8V for 1.8V signaling or +3.3V for 3.3V signaling. The total load on the 28 GPIO pins should not exceed 50mA. GPIO_VREF must be powered for the CM4 to start correctly.

2.5.1. Alternative function assignments

Up to six alternative functions are available for GPIO pins. Table 1 provides an overview of these assignments, with a detailed legend in Table 2.

Table 2. GPIO pins alternative function legend

^a The Broadcom serial control bus is a proprietary bus compliant with the Philips® I2C bus/interface.
^b BSC master 2 & 7 are not user-accessible.
^c SPI 2 is not user-accessible.

2.6. Dual HDMI 2.0

The CM4 supports two HDMI 2.0 interfaces, capable of driving 4K images. When both outputs are used, each can support up to 4Kp30. If only HDMI0 is used, resolutions up to 4Kp60 are possible. HDMI signals should be routed as 100Ω differential pairs, with signal lengths within a pair matched to better than 0.15mm. Pair-to-pair matching is less critical, typically needing to be within 25mm.

CEC is also supported, with an internal 27kΩ pull-up resistor. Basic on-board ESD protection is provided for I2C EDID and CEC signals. Unlike the Raspberry Pi 4 Model B, the CM4's HDMI signals do not have additional ESD protection, which may be required depending on the application.

2.7. CSI-2 (MIPI serial camera)

The CM4 supports two camera ports: CAM0 (2 lanes) and CAM1 (4 lanes). CSI signals require 100Ω differential pair routing, with signal lengths within a pair matched to better than 0.15mm. Documentation for the CSI interface is available on the Raspberry Pi website, and Linux kernel drivers can be found on GitHub.

Note: The official Raspberry Pi firmware supports OmniVision OV5647, Sony IMX219, IMX296, IMX477, and IMX708 camera sensors. No security device is needed for these sensors on Compute Module devices.

2.8. DSI (MIPI serial display)

The CM4 supports two display ports: DISP0 (2 lanes) and DISP1 (4 lanes), each lane supporting a maximum data rate of 1Gbps. While Linux kernel drivers exist, the DSI interface is not fully documented. Only DSI displays supported by official Raspberry Pi firmware are compatible. DSI signals require 100Ω differential pair routing, with signal lengths within a pair matched to better than 0.15mm.

Note: Although only official DSI displays are supported, other displays can be connected via the parallel DPI interface (a GPIO alternative function). The CM4 can support up to three displays simultaneously, regardless of type (HDMI, DSI, DPI).

2.9. I2C (SDA0 SCL0)

This internal I2C bus is typically allocated to CSI1 and DSI1 for firmware control. It can be used as a general I2C bus if CSI1 and DSI1 are not in use or are controlled by the firmware. For instance, libcamera runs on the ARM without using firmware, allowing CSI1 and this I2C bus to be utilized. SDA0 connects to GPIO44 and SCL0 to GPIO45 on the BCM2711.

2.10. I2C (ID_SD ID_SC)

This I2C bus is primarily used for identifying HATs and controlling CSI0 and DSI0 devices. If the firmware does not use the I2C bus (e.g., CSI0 and DSI0 are inactive), these pins can be repurposed as GPIO 0 and GPIO 1.

Note: If using these pins as GPIO, add force_eeprom_read=0 and disable_poe_fan=1 to config.txt to prevent the firmware from checking for a HAT EEPROM.

2.11. SDIO/eMMC (CM4Lite only)

The CM4Lite lacks on-board eMMC. Its eMMC signals are available on the connector, allowing the use of an external eMMC or SD card. The SD_PWR_ON signal controls an external power switch for the SD card; it is typically not used for eMMC. For SD card booting, a pull-up resistor is needed to default the power switch to the 'on' state. SD_VDD_OVERRIDE set high (3.3V) forces 1.8V signaling on the SDIO interface, commonly used with eMMC.

2.12. Analog IP0/IP1

These are two spare inputs on the MXL7704. Consult the MXL7704 datasheet for their usage. On-board filtering is provided by a 100nF capacitor to ground for each signal. On the Raspberry Pi 4 Model B, these pins are connected to the USB C connector's CC1 and CC2 pins.

2.13. Global_EN

Pulling this pin low initiates the CM4's lowest power-down state. After software shutdown, Global_EN must be held low for over 1ms to restart the CM4's power system.

Tip: It is recommended to pull this pin low only after the OS has shut down.

2.14. RUN_PG

This pin signals when the CM4 has started (high). Driving it low resets the module. Use with caution, as open files on a filesystem may not be closed.

2.15. nRPI_BOOT

During boot, if this pin is low, eMMC booting is halted, and booting is switched to rpi boot via USB.

2.16. LED_nACT

This pin drives an LED to mimic the green LED on the Raspberry Pi 4 Model B. Under Linux, it flashes to indicate eMMC access. Error conditions during boot are signaled by an error pattern flash, which can be decoded using tables on the Raspberry Pi website.

2.17. LED_nPWR

This pin requires buffering to drive an LED and replicates the red power LED on the Raspberry Pi 4 Model B.

2.18. EEPROM_nWP

It is recommended to pull this pin low in final products to prevent end-users from altering the on-board EEPROM contents. Refer to the Raspberry Pi 4 Model B documentation for instructions on EEPROM write protection settings.

Chapter 3. Electrical and mechanical

3.1. Mechanical

The CM4 is a compact module measuring 40mm x 55mm. Its depth is 4.7mm, increasing to 5.078mm or 6.578mm when connected, depending on the stacking height. It features 4 x M2.5 mounting holes (3.5mm inset from the edge) and a PCB thickness of 1.2mm ± 10%. The BCM2711 SoC height, including solder balls, is 2.378mm ± 0.11mm.

Stacking height options:

1. 1.5mm with mating connector (0mm clearance under CM4): DF40C-100DS-0.4v
2. 3.0mm with mating connector (1.5mm clearance under CM4): DF40HC(3.0)-100DS-0.4v

If the on-board wireless antenna is used, it must be oriented towards the enclosure's edge. Nearby metal should have cut-outs to avoid degrading wireless performance. A minimum 10mm clearance around the PCB antenna is suggested, but designers should verify performance.

Note: Component placement and arrangement may vary slightly due to revisions, but maximum component heights and PCB thickness remain as specified. A step file of the CM4 is available for guidance.

For optimal wireless performance, avoid placing metal, including ground planes, under the antenna. A ground plane cutout of at least 6.5mm x 11mm (ideally 8mm x 15mm) is recommended. Failure to meet these requirements may degrade wireless performance, especially in the 2.4GHz band. Using an external antenna is advised where possible.

3.2. Thermal

The CM4 dissipates less power than the Raspberry Pi 4 Model B and has less metal and fewer connectors, resulting in reduced passive heat sinking. Despite lower power consumption, it may run warmer.

The BCM2711 processor throttles its clock rate to maintain an internal temperature below 85°C. In high ambient temperatures, clock throttling may occur. If the processor cannot lower its internal clocks sufficiently, its case temperature may exceed 85°C. Thermal solutions should ensure ambient temperatures for other silicon devices on the CM4 remain within the operating range.

Operating temperature range: -20°C to +85°C (non-condensing). Optimal RF wireless performance is between -20°C and +75°C.

3.3. Electrical specification

Warning: Exceeding the absolute maximum ratings may cause permanent damage. These are stress ratings only; functional operation outside these limits is not implied, and prolonged exposure may affect reliability.

Note: V_{GPIO_VREF} is the GPIO bank voltage and must be tied to either the 3.3V or 1.8V rail of the CM4.

Note: The figures in Table 5 depend significantly on the end application.

Chapter 4. Pinout

All ground pins should be connected. If none of the signals on the second connector (pins 101 to 200) are used, the connector may be omitted to reduce costs, but mechanical stability must be considered.

GPIO pins 0-27 must not exceed CM4_3.3V for +3.3V signaling or CM4_1.8V for +1.8V signaling. These pins are identical to those on the Raspberry Pi 4 Model B's 40-pin connector.

If the CM4_1.8V rail powers devices other than GPIO_VREF, ensure that in case of surprise power removal (e.g., +5V pin drops below +4.5V), the load on CM4_1.8V goes to zero. Similarly, if CM4_3.3V powers devices other than GPIO_VREF, ensure it never falls below CM4_1.8V during surprise power removal. This is typical, but verify in your design. If CM4_3.3V falls below CM4_1.8V, extra circuitry is needed to disconnect the CM4_3.3V load.

Do not apply reverse voltage to any pin, as it may prevent subsequent power-ups.

4.1. Differential pairs

It is recommended that P/N signals within a pair are matched to better than 0.15mm. Pair-to-pair matching is less critical, with HDMI pairs typically needing matching within 25mm.

4.1.1. 100Ω differential pair signal lengths

On the CM4, all differential pairs are matched to better than 0.05mm (P/N signals).

Note: It is recommended that pairs are also matched on the interface board.

Pair-to-pair matching may not always be precise, as many interfaces do not require high accuracy. Table 7 details CM4 track-length differences within groups, where a non-zero value indicates how much longer a track is in mm compared to the signal with zero length difference.

4.1.2. 90Ω differential pair signal lengths

On the CM4, all differential pairs are matched to better than 0.05mm (P/N signals).

Note: It is recommended that pairs are also matched on the interface board.

Pair-to-pair matching may not always be precise, as many interfaces do not require high accuracy. Table 8 details CM4 track-length differences within groups, where a non-zero value indicates how much longer a track is in mm compared to the signal with zero length difference.

Chapter 5. Power

5.1. Power-up sequencing

The CM4 requires a single +5V supply and can provide up to 600mA at +3.3V and +1.8V to peripherals. All pins should remain unpowered until the +5V rail is applied. For EEPROM write protection, EEPROM_nWP should be low before power-up. For USB booting, RPI_nBOOT must be low within 2ms of the +5V rising edge.

The +5V rail must rise monotonically to at least 4.75V and remain above it for the CM4's operation. The power-up sequence initiates when both the +5V rail exceeds 4.75V and GLOBAL_EN rises. GLOBAL_EN has an internal RC delay, ensuring it rises after +5V.

The sequence of events is:

1. +5V rises
2. GLOBAL_EN rises
3. +3.3V rises
4. +1.8V rises (at least 1ms after +3.3V)
5. RUN_PG rises (at least 10ms after +1.8V)
6. EXT_nRESET rises (at least 1s after RUN_PG)

5.2. Power-down sequencing

The operating system should be shut down cleanly before removing power to ensure file system consistency. Filesystems like btrfs, f2fs, or overlayfs are recommended if clean shutdowns are not guaranteed. After OS shutdown, the +5V rail can be removed, or GLOBAL_EN can be pulled low to enter the CM4's lowest power mode. During shutdown, the +1.8V rail discharges before the +3.3V rail.

5.3. Power consumption

Power consumption varies with the CM4's workload. The lowest shutdown power consumption (GLOBAL_EN low) is approximately 15µA. With GLOBAL_EN high and software shut down, consumption is typically 8mA. Idle power consumption is around 400mA, varying with the OS. Operating power consumption is approximately 1.4A, also dependent on the OS and tasks.

5.4. Regulator outputs

The CM4 includes on-board regulators providing +3.3V and +1.8V, each capable of supplying 600mA to connected devices. These regulator outputs are not included in the power consumption figures.

Appendix A: Troubleshooting

The CM4 undergoes several power-up stages. If an error occurs at any stage, power-up halts.

Hardware checklist

1. Verify the +5V supply: Pull GLOBAL_EN low, apply a 2A load to the +5V supply, and check if it remains above +4.75V (ideally > +4.9V) under load and noise.
2. Remove the 2A load, keeping GLOBAL_EN low.
3. Check if the CM4 +3.3V rail is below 200mV. A higher reading indicates external power back-feeding the CM4, possibly via digital pins or Ethernet.
4. With GLOBAL_EN still low, check if the CM4 +1.8V rail is below 200mV. A higher reading suggests external back-feeding of the 1.8V rail. (Ignore if no pins are connected).
5. Remove the pull-down on GLOBAL_EN.
6. Confirm GLOBAL_EN now goes high (it is internally pulled up).
7. Check if the +3.3V supply rises above +3.15V. If not, there might be excessive load on the +3.3V rail.
8. Check if the +1.8V rail rises above +1.71V. If not, there might be excessive load on the +1.8V rail.
9. Verify RUN_PG goes high.
10. Check if ACT_LED starts oscillating, indicating booting. Ensure it's not flashing an error code.

Bootloader

1. Connect an HDMI cable to view the HDMI diagnostics screen.
2. Connect a USB serial cable to GPIO pins 14 and 15. Refer to: https://www.raspberrypi.com/documentation/computers/configuration.html#configuring-uarts
3. Short the nRPIBOOT pin to ground to force USB boot mode. The CM4IO board has a jumper for nRPIBOOT to enable different boot modes (e.g., network) and UART logging. Refer to: https://www.raspberrypi.com/documentation/computers/compute-module.html#flashing-the-compute-module-emmc

rpi-eeprom-update

The CM4 does not run recovery.bin from eMMC (or SD Card on CM4Lite). Bootloader EEPROM updates are performed via usbboot or self-update.

EEPROM write-protect

The on-board EEPROM can be write-protected by shorting EEPROM_nWP to ground. The CM4IO board has a jumper for EEPROM_nWP.

Refer to: https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#raspberry-pi-4-bootloader-configuration

Firmware

1. A kernel version 5.4 or newer and the latest firmware release are required. Updates can be performed using usbboot to mount the eMMC as a USB MSD device.
2. Nightly OS images are available, containing the rpi-update master firmware + kernel. CM4 bug fixes are typically included in these images, except for test/patch binaries. See: http://downloads.raspberrypi.org/nightlies/

Kernel

1. Updated OS images use the new Raspberry Pi Compute Module 4 device tree file. If not found, the Raspberry Pi 4 Model B device tree file will be used. See: https://github.com/raspberrypi/linux/blob/rpi-5.4.y/arch/arm/boot/dts/bcm2711-rpi-cm4.dts

Appendix B: Availability

Support

For documentation, consult the Compute Module Hardware documentation section on the Raspberry Pi website. Support questions can be posted on the Raspberry Pi forum.

Ordering codes

Table 9. Part number options

Table 10. Ordering options

Note: RRP was correct at time of publication and excludes taxes.

Packaging

Small quantities are supplied in individual cardboard boxes with internal ESD coating, eliminating the need for a separate ESD bag. This packaging is recyclable.