M-PHY benefits and challenges

Power
A mobile terminal PHY’s real value is largely tied to its ability to be a power miser. In the case of the M-PHY minimizing power is accomplished by:

• Power gradation through multiple states and extensive use of save states, as shown in Figure 3
• Fast and simple transition between all states
• Fast wake up time in LS mode without the need for synchronization or CDR
• Aggressive power targets for both HS and LS operation. See Figure 4
• System is optimized for send and stop strategy, with NOP/Filler use, as shown in Figure 5.

Figure 3: M-PHY states and power grading / Source: STE


Figure 4: M-PHY aggressive power target for HS and LS operation / Source: STE


Figure 5: Send & Stop strategies / Source: STE

The M-PHY makes efficient use of many different modes of operations: unpowered, disable, hibernate (Hibern8), low-speed mode and high-speed mode. The state machine is shown in Figure 6 below.
Disable mode is the lowest power mode entered into once the power supply is turned on. Hibern8 is an ultra low power state, which can be used without configuration loss. It enables online wake up capability without any side band signals (in Type-I). The transition to another M-PHY state takes hundreds to thousands of microseconds. That recovery time is programmable to fit the application requirements.

The LS and HS modes each define power saving states, Sleep and Stall respectively in addition to their respective burst states. LS-Mode is used when the application requires low activity level. Power saving is accomplished since no PLL, CDR, or synchronization is required. This not only saves power but also minimizes transition time, which is typically in the range of microseconds. 

HS-Mode targets power numbers in the order of pJ/bit. In this mode, the M-PHY delivers data with an aggressive total power target of 20 mW. Data recovery is required to recognize bit information at Gbps speed. A synchronization sequence locks the receiver CDR phase and frequency in an extremely short time. The M-PHY specification does not dictate this sequence length, instead that is determined by the application. This allows each application to configure and use the M-PHY in the most optimal way. HS-MODE utilizes the power-saving Stall state to reduce the power consumption by stopping lane activity and any unnecessary power dissipation, while offering a fast transition in the range of ns.


Figure 6: The M-PHY state machine / Source: MIPI Alliance (Click figure for larger image)

Programmable amplitude and switchable termination
To save as much power as possible, in addition to the multiple low power modes employed, the M-PHY supports both programmable low amplitude and switchable termination. There are two amplitude settings, large amplitude (LA) and small amplitude (SA). The M-PHY can also operate terminated, or un-terminated, as shown in Figure 7. For LS operation, SA un-terminated mode can be used to minimize power. HS operation support of terminated operation is mandatory, while support for un-terminated operation is optional.

Figure 7: The M-PHY Transceiver example / Source: MIPI Alliance