Apple Mac Logic Board PM_SLP_S4_L Signal Timing and Voltage

This article is for MacBook logic board component-level repair professionals, written by IT-Tech Online, the MacBook repair specialist in Melbourne, Australia.

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We will discuss one of the important MacBook control signal PM_SLP_S4_L  and PM_SLP_S4_L voltage here. The PM_SLP_S4_L signal will transform a MacBook from the  S5 state to the S3 state. We will use the 13" MacBook Air 2015-2017 logic board schematics 820-00165 as references.

When will the CPU or PCH shoot out the PM_SLP_S4_L signal?

This question burdens a lot of component level repair technicians. This article discusses the prerequisites that must be met before the Intel CPU/PCH outputs PM_SLP_S4_L. In order to understand the PM_SLP_S4_L signal, we need to know the Intel CPU/PCH all in one design. Once you understand the PM_SLP_S4_L timing, a logic board with missing PM_SLP_S4_L signal issue is not hard to deal with.

There are two sets of PM_SLP_S4_L prerequisites. Intel 1st to 5th generation CPUs use the same set of prerequisites called 11+1 conditions. Intel 6th generation onward CPUs use a different set of prerequisites called 13+1 conditions (discuss in a separate article). Intel and Apple use a different naming system. We use the Apple naming system by default. Since Apple uses different names referring to the same Intel CPU/PCH signal in different models, we use MacBook Air 2015 A1466  Haswell-ULT CPU 820-00165 schematics here as an example.

Intel 1st to 5th generation CPUs  11+1 prerequisites for PM_SLP_S4_L:

Apple Name Notation:



 Intel Name Notation:


7. PP3V3_S5          (VCCDSW3-3)
9. PP3V3_SUS   (VCCSUS3)

If PM_SLP_S4_L missing, we need to check these before replacing the CPU/PCH.

  • 1. PPVRTC_G3H (VCCRTC) – real-time clock circuit power supply. PPVRTC_G3H voltage is 3.3V.
  • 2. RTC_RESET_L (RTCRST#) – real-time clock circuit reset signal. RTC_RESET_L voltage is 3.3V.
  • 3. PCH_SRTCRST_L (SRTCRST#) secondary real-time clock circuit reset signal. PCH_SRTCRST_L voltage is 3.3V.
  • 4. PCH_INTVRMEN_L (INTVRMEN) – Intel CPU/PCH internal voltage regulators 1.05V and 1.5V enable signal. PCH_INTVRMEN_L voltage is 3.3V.
  • 5. PCH_DSWVRMEN_L (DSWVRMEN) – Intel CPU/PCH deep sleep well(power) voltage regulator enable signal. PCH_DSWVRMEN_L voltage is 3.3V.
  • 6. SYSCLK_CLK32K_RTCX1 (RTCX1) – clock signal for RTC circuit. It can be measured by an oscilloscope. The SYSCLK_CLK32K_RTCX1 clock frequency is 32KHz.
  • 7. PP3V3_S5  (VCCDSW3-3)  – deep sleep mode power rail. PP3V3_S5 voltage is 3.3V.
  • 8. PM_DSW_PWRGD (DPWROK) – deep sleep mode power good signal. PM_DSW_PWRGD voltage is 3.3V.
  • 9. PP3V3_SUS (VCCSUS3)  suspended mode(shallow sleep mode) power rail. PP3V3_SUS voltage is 3.3V.
  • 10. PM_BATLOW_L (BATLOW#) – SMC monitors the status of the battery. If the battery voltage is low, SMC will prevent the MacBook from power-up. PM_BATLOW_L voltage is 3.3V.
  • 11. PM_RSMRST_L (RSMRST#)  – It stands for “resume reset”. It means the system is ready for resuming from suspended mode (sleep mode) to power on mode. PM_RSMRST_L voltage is 3.3V.
  • 12. PM_PWRBTN_L (PWRBTN#) – power button on/off control signal. It is a  high-low-high pulse signal. PM_PWRBTN_L voltage is 3.3V. If the logic board is connected to the battery only, you need to press the power button on the keyboard to send the pulse signal to the CPU/PCH (via SMC). If you connect the logic board to the charger without connecting the battery,  once SMC receives the SMC_BC_ACOK from charging circuit power management IC u7100, SMC will output SMC_ADAPTER_EN (ACPRESENT). In this case, the signal SMC_ADAPTER_EN will act as the same as PM_PWRBTN_L. In other words, the PM_PWRBTN_L is not always a “must" in some situation. It is why we call it “+1" condition.

When the 11 prerequisites are met, once you press the power button (PM_PWRBTN_L) or connect a charger to the logic board (SMC_ADAPTER_EN), CPU/PCH will shoot out PM_SLP_S5_L, PM_SLP_S4_L and PM_SLP_S3_L to enable S4, S3 and S0 state power rails. 

In most models, Apple does not utilise PM_SLP_S5_L, just feedback this signal to SMC and that is all. PM_SLP_S4_L will enable the S3 state power rails including the memory module power rail. PM_SLP_S3_L will enable the rest S0 state power rails.

PM_SLP_S4_L voltage

PM_SLP_S4_L is not a power rail, it is a control signal output by the PCH or CPU. The PM_SLP_S4_L signal voltage is 3.3V. PM_SLP_S5_L and PM_SLP_S3_L signal voltages are all 3.3V as well. PM_SLP_S4_L plays a very important role in MacBook power-up process. It is a power management control signal being used to transit the Macbook from standby state (S5) to power-on state (S0). A lot of MacBook power issues are related to this PM_SLP_S4_L signal missing.

Lists of other important power rails and control signals and their voltage

PP5V_S3 power rail voltage is 5V

PP3V3_S3 power rail voltage is 3.3V

PP1V8_S3 power rail voltage is 1.8V

PP1V2_S3 power rail voltage is 1.2V

VBUS power rail voltage is 5V

PP1V05_S0 power rail voltage is 1.05V

PP1V5_S0 power rail voltage is 1.5V

PM_SLP_S3_BUF_L control signal voltage is 3.3V

ALL_SYS_PWRGD control signal voltage is 3.3V

SMC_DELAYED_PWRGD control signal voltage is 3.3V

PM_PCH_PWROK control signal voltage is 3.3V

 PCH_PWROK control signal voltage is 3.3V

CPU_VCCST_PWRGD control signal voltage is 1.05V

PM_PCH_SYS_PWROK control signal voltage is 3.3V 

PLT_RESET_L control signal voltage is 1.05V

In the following case studies of Mac logic board repair, we will apply this PM_SLP_S#_L timing knowledge in each case to quickly identify the problems. Say goodbye to the old trial-and-error problem-solving approach, adop this advanced “measure, think, pinpoint and replace" approach.  You will not only increase the success rates but also build up solid knowledge on logic board repair.  This knowledge, in turn, will increase productively.

Case Study 1

MacBook Model: A1278 2011 820-2936

Fault Description: water damaged, no power, PP3V3_S5 present, PP5V_S3  missing.

Diagnosis: Trace  PP5V_S3 enable signal P5VS3_EN_L = 3.3V, not ok.  It should be low (EN_L means low-enable). Trace P5VS3_EN_L to Q7911 and find out the PM_SLP_S4_L voltage is 0V.

File name : 2011-Macbook-Pro-A1278-pm_slp_s4_l.pdf

Analysis: The 11+1prerequisites must be met before the Intel CPU/PCH outputs PM_SLP_S4_L. If all the prerequisites are ok, then we will consider replacing the PCH u1800. The further measurement indicates PPVRTC_G3H is 1.5V, not ok. It should be 3.3V. The suspect chips are:

1. the clock chip u2800 output low

2. the u1800 PCH pulls down the voltage (needs PCH replacement, expensive repair)

3. resistors on the PPVRTC_G3H line R1800, R1801, R1802 R1803…. pull down the voltage

File name : 2011-Macbook-Pro-A1278-system-clock-chip.pdf

Micro Soldering: We spot watermarks (very little mark, the board must have been ultrasonically cleaned by someone) on the clock chip u2800, so we replace it first. After that, the board is fully functional. In a situation like this, we always replace the easy one first. If replacing the clock chip u2800 did not solve the problem, we will remove the resistors on the PPVRTC_G3H power line before replacing the PCH. Replacing the PCH needs a BGA rework station and related BGA rework skills and the cost the repair will be much higher.

Learn By Example: If we did not perform the measurement before replacing the suspected  u2800, we will get this statement: “The faulty clock chip u2800 caused PM_SLP_S4_L missing." If we did the measurement before replacing u2800, we will get a  much more precise statement: “The clock chip u2800  did not output enough voltage to power up PCH RTC circuit, so the PCH will not shoot out PM_SLP_S4_L". Next time if we encounter a similar situation that the PCH  or the resistors on the PPVRTC_G3H line pull down the voltage and cause PM_SLP_S4_L missing,   we will handle the situation with ease. A statement like"The faulty clock chip u2800 caused PM_SLP_S4_L missing" is never enough for the advanced technicians and engineers.

Five of the twelve prerequisites are related to the clock chip, PPVRTC_G3H, RTC_RESET_L, PCH_SRTCRST_L, PCH_INTVRMEN_L and SYSCLK_CLK32K_RTCX1. So the clock chip should be the first chip to check if you get a PM_SLP_S4_L missing logic board. Newer models 2016 onward use a new clock chip SLG3AP3444 and it no longer outputs power rail to RTC circuit. But no matter what chip is used, we still can trace back to it by searching the PPVRTC_G3H (VCCRTC) power rail. Understand the principle and build up a personal knowledge database gradually is a “must" for a component level repair engineer. 

Case Study 2

MacBook Model: A1502 2015 820-4924

Fault Description: water damaged, no power, PP3V3_S5 present, PP5V_S3  missing.

Diagnosis: Trace  PP5V_S3 enable signal P5VS3_EN = 0V not ok. It should be high 3.3V. Trace P5VS3_EN to R8175 and R8179 and find out the PM_SLP_S4_L is missing (0V).

File name : 2015-Macbook-Pro-A1502-pm_slp_s4_l.pdf

Analysis: Check the 11+1prerequisites, all are good except the PM_BATLOW_L is low 0.9V. This signal is from SMC. If SMC detects a battery is connected, it will use SMBUS_BATT_SCL/SDA to get the status of the battery. If the battery voltage is lower to a preset value, SMC will keep  PM_BATLOW_L as low until the battery is charged and the voltage is above the preset value. If SMC detects no battery connected, it will assume the battery is ok and output PM_BATLOW_L as high (open-drain) immediately. PM_BATLOW_L is pulled up by R1410 10K resistor and the S5 rail PP3V3_S5_PCH_GPIO. 

File name : 2015-Macbook-Pro-A1502-PM_BATLOW_L.pdf

So the suspect chips are:

1. R1410 open line or PP3V3_S5_PCH_GPIO not present on R1410

2. Q3000 pulls down the voltage

3. SMC u5000 pulls down the voltage (open drain function failed)

4. CPU u0500 pulls down the voltage (faulty CPU, replacing CPU will be expensive)


1. Measure R1410 10K, ok. PP3V3_S5_PCH_GPIO present on R1410 pin 1, ok. 

2. Remove Q3000, PM_BATLOW_L is still 0.9V.

3. Remove SMC u5000, connect a 3.3V external power to pin 1 of R1410, PM_BATLOW_L is 3.3V  and ok now. This result means the SMC pulls down the voltage and the CPU is OK. We need a 3.3V external power for measurement as once we remove the SMC, we will not get the S5 rail PP3V3_S5 (PP3V3_S5 is controlled by SMC_PM_G2_EN output from SMC).

4. Put back Q3000, get an SMC from a donor board, re-ball and solder back to the board. After that, the board is fully functional.

Case Study 3

MacBook Model: A1466 2015 820-000165

Fault Description: no power, very clean board no water damage, PP3V3_S5 present, PP5V_S3  missing.

Diagnosis: Trace  PP5V_S3 enable signal P5VS4RS3_EN = 0V, not ok. It should be high 3.3V. Trace P5VS4RS3_EN to  R8179 and find out the PM_SLP_S4_L is missing (0V).

Analysis: Check the 11+1prerequisites, all are good except the PP3V3_SUS is low 0.1V. This power rail is from the 3.3V sus switch chip u8020.

File name : 2015-macbook-air-A1466-PP3V3_SUS.pdf

Repair: Check u8020 input PP3V3_S5 is 3.3V, ok. Check u8020 enable signal  P3V3SUS_EN is 3.3V, ok.  So u8020 must be faulty. After replacing u8020, we have a fully functional board. The job was done in 10 munites.  This is a very easy one, if you understand the principle. 

Case Study 4

MacBook Model: A1466 2013 820-3437

Fault Description: no power, water damage, PP3V3_S5 present, PP5V_S3  missing.

Diagnosis: All the11+1prerequisites are present, but the PCH/CPU does not shoot out PM_SLP_S4_L.

Analysis: Time to replace the PCH/CPU?  NOT YET! If we cannot measure PM_SLP_S4_L on our multimeter, it doesn’t mean it does not exist. Here is the PM_SLP_S4_L signal shown on an oscilloscope.

PCH/CPU shoots out PM_SLP_S4_L at 800ms (timeline) after PPBUS_G3H had been created and stabilised. The PM_SLP_S4_L voltage is 3.3V, perfect.  But it drops to 0V at 920ms. The PM_SLP_S4_L only exits for 120ms. This is why the multimeter did not pick it up. 

Repair: R6120 is corroded and the SPI_MLB_CS_L  signal cannot pass through. At timeline 920ms, the PCH fails to read data from SPI chip u6100 to determine the clocks and other user-definable settings, therefore, pulls down PM_SLP_S4_L. Replace the R6120 we have a fully functional board.

File name : 2015-Macbook-Air-A1466-SPI_MLB_CS_L.pdf

Question: SPI_MLB_CS_L is S0 state signal and PM_SLP_S4_L is S4 state signal. How come the S0 signal affects the S4 signal? This question was asked in the last section of Louis Rossmann’s video “MacBook Air logic board repair PM_SLP_S4_L missing"? We will discuss this later.

Next article:  MacBook logic board power rails and ALL_SYS_PWRGD


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