M.2 pinout: Key M, Key B and Key E sockets

PET or PER: reading pair direction without a mistake

The most expensive M.2 trap is a vocabulary problem. The congatec table, which follows the specification, names the PCIe pairs from the host point of view: the card transmits on PERn0/PERp0 (pins 41/43) and receives on PETn0/PETp0 (47/49). The Intelligent Memory and UD info SSD datasheets, or the SIMCom SIM8200EA-M2 module, print PETn0 on pin 41 instead, because they speak from the card side.

The electrical assignment does not change: every source we checked puts the card transmitter on 41/43 (Quectel RM520N-GL: PCIE_TX on 41/43; Seeed Jetson carrier table: host receive on 41/43). Before routing, we therefore check each pair from the direction column, never from the name alone. The same flip affects the Key E UART and PCM: Embedded Artists calls pin 22 UART TXD, which congatec and Panasonic call UART RXD.

Another SATA trap: the positive B pair (SATA-B+) shares pin 41 with PERn0, a negative pin in PCIe. The Apacer P201-M SATA SSD datasheet confirms the placement: host receive positive on 41, negative on 43, host transmit negative on 47, positive on 49.

Keys B, M and E: what fits which socket

The key is a notch in the card that matches a ridge in the connector. A B+M card has both notches: the Apacer datasheet states it therefore plugs into a Key B socket as well as a Key M socket. A Key M only card, typical of PCIe x4 NVMe SSDs, does not fit a Key B socket.

The pinout explains what a B+M card can do: the Key B socket only wires PCIe lanes 0 and 1 (41 to 49 and 29 to 37), while the Key M socket wires four. A B+M card therefore runs SATA or PCIe x2 at most. Key E, finally, is for connectivity cards: Embedded Artists describes its Wi-Fi and Bluetooth module as a 2230-D5-E.

On a Key B socket, the host reads CONFIG_0 to CONFIG_3 (pins 21, 69, 75 and 1) to learn which interface the card uses: each pin is grounded or left open on the card, with a pull-up on the host, readable even with the card unpowered. Four grounds mean a SATA SSD, four high pins mean no card (congatec AN43 decode table). The SIMCom SIM8200EA-M2 module grounds CONFIG_0, CONFIG_1 and CONFIG_2 and leaves CONFIG_3 open: WWAN over PCIe and USB 3.

The Key M socket has no CONFIG pins: it relies on PEDET (pin 69). Per the Linux kernel documentation, which cites the M.2 specification r4.0, PEDET low means a SATA card and high means a PCIe card. A socket that only handles NVMe will never run an M.2 SATA SSD, even though it fits mechanically.

Sizes 2230, 2242, 2280 and connector heights

An M.2 card name gives its width then its length in millimeters, then the component height class and the key. Embedded Artists spells out 2230-D5-E: 22 mm wide, 30 mm long, components up to 1.5 mm on both sides, Key E. A 2242 card is therefore 22 × 42 mm and a 2280 is 22 × 80 mm, the size of the Intelligent Memory NVMe SSD. Modems are wider: 30 × 42 mm for the Quectel EM05, 30 × 52 mm for the RM520N-GL.

The connector follows the card thickness. TE Connectivity gives a 2.25 mm height for cards with components on one side and 3.2 mm for double-sided cards, at 0.5 mm pitch. The far end of the card is screwed to a standoff or threaded insert: Panasonic calls for an M2.5 × 4 screw and a card parallel to the host board. We therefore pick the connector height and the standoff height together, for the card length planned.

The notch is off center. On the UD info M2P-80DE (Key M) and Apacer P201-M (B+M) SSD drawings, each notch is 1.20 ± 0.05 mm wide; the Key M notch axis sits 6.125 mm from the card axis, the Key B notch axis 5.625 mm on the other side. These dimensions fall exactly in the middle of positions 59 to 66 and 12 to 19, at 0.25 mm per pin number. The 42 mm length of a 2242 card comes from the ADATA IM2S3334 datasheet.

3.3 V supply and 1.8 V logic levels

Key M and Key E sockets take 3.3 V only (nine pins on Key M, four on Key E). The Key B socket reads 3.3V/VBAT: the Quectel and SIMCom 4G/5G modules accept 3.135 to 4.4 V and draw large current bursts when they transmit. For the RM520N-GL, Quectel asks for a supply able to deliver 3.0 A continuously, two 220 µF low-ESR capacitors and a VCC trace at least 3 mm wide; SIMCom gives 2.7 A peak for the SIM8200EA-M2.

Sideband signals do not all share one level. Per congatec, PERST#, CLKREQ#, PEWAKE#, SUSCLK, W_DISABLE# and the LEDs are 3.3 V; SMBus, I2C, ALERT#, UART, PCM, SDIO, COEX and the Key B GPIOs are 1.8 V. Connecting a 3.3 V processor GPIO to a 1.8 V module pin needs a level shifter.

PERST#, CLKREQ#, PEWAKE#, SUSCLK and W_DISABLE#

PERST# is the PCIe functional reset, driven by the host. CLKREQ# and PEWAKE# are open-drain outputs of the card: SIMCom requires an external pull-up on the host, and Quectel notes their level depends on the host pull-up voltage. Per UD info, CLKREQ# also serves the L1 power-management substates. Quectel advises driving active-low inputs from a push-pull GPIO rather than a pull-down resistor, to avoid a divider with the module internal pull-up.

SUSCLK is a 32.768 kHz suspend clock supplied by the host to the card (Linux kernel documentation). Embedded Artists rates it "always needed" for its Wi-Fi and Bluetooth module: plan a 3.3 V 32.768 kHz oscillator. Beware on Key B: SIMCom reuses pin 68 as I2C_SDA.

W_DISABLE1# turns Wi-Fi off and W_DISABLE2# turns Bluetooth off on Key E; on Key B, W_DISABLE1# puts the modem in airplane mode. These inputs are active low: the Quectel and SIMCom modules pull them up internally. FULL_CARD_POWER_OFF# does the opposite: both modules pull it down, so a modem whose host leaves that pin floating never starts.

Routing the PCIe lanes of an M.2 socket

For PCIe, Quectel sets 85 Ω ± 10 % differential impedance, intra-pair length matching under 0.7 mm and a total length under 200 mm; for USB 2.0 and USB 3, 90 Ω. The coupling capacitors sit at the transmitter: those of the module transmitter are inside the module, those of the host transmitter go near the host. The differential impedance calculator gives the trace geometry for your stack-up.

On Key M, lane 0 is on 41 to 49 and lane 3 near pin 5; the physical lane order must match the controller, unless it supports lane reversal. We then measure the eye diagrams on our Tektronix oscilloscope with TekExpress, which covers PCI Express, SATA and USB compliance.