We need to provide power, clock and reset signals to kick-start the SMC chip.
pp3v42_g3h created by u7090 will provide main power to the SMC chip u5000 via the BGA ball K12, VBAT. The pp3v42_g3h also goes through L5001 to power the SMC internal logic model VDDA. The SMC will create pp1v2_s5_smc_vddc to power itself. Then the SMC crystal circuit will start to output 12 MHZ square-ls clock for the SMC.
U5110 monitors the voltage of pp3v42_g3h. If pp3v42_g3h is not less than 3.0V and stable, U5100 will output a delayed smc_reset_l 3.3V to reset the SMC chip. The delay time is determined by R5101and C5101. U5110 also outputs pp3v3_s5_avref_smc 3.3V references voltage for the analog-digital conversion function of SMC sensors circuits including current sensors, voltage sensors and temperature sensors.
On receiving smc_bc_acok from charging chip u7100, SMC will communicate with the MagSafe via the one-wired circuit and the green light on the charger will be on. Meanwhile, SMC uses SMBus to communicate with u7100 to adjust the ppbus_g3h from base voltage 8.1V to standard 8.45V. If you can get 8.45V ppbus_g3h output, it means SMC is functioning, at least partially. SMC uses the same SMBus to read battery status to determine the battery charging current. SMS monitors system input current (AMON) and battery charging current (BMON) all the time.
SMC is a very important and complex chip designed by Apple. There is no “new" SMC chip you can buy. It has to be from a donor board with the same model and year. We will discuss more SMC functions later.
SMC requires VDD (PP3V42_G3H), VDDA (PP3V3_S5_SMC_VDDA) and SMC_RESET_L to be 3.42V in order to turn on. It also requires the crystal to be in good shape to generate the clock. The crystal circuit is almost never the problem as long as it is not visually damaged. VREFA (PP3V3_S5_AVREF_SMC) is not required for the SMC to turn on, but it is required to reach a stable S0 state, otherwise without current/voltage sensing working properly the board won't reach an S0 state or cycle. In general the board can still boot in SMC bypass mode.
SMC bypass is a mode where several SMC features are disabled (sensors in particular), you disconnect all power, press power button, connect Magsafe and keep power button held for 5 to 10 seconds. Fans should go at max speed.