After performing the phase reference measurements, measure the phase of the speaker load. This is performed in the same manner as the reference measurements, except the loopback is disabled in bit 7 register 0x16. Previously, the phase reference is measured on channel 1and channel 3. In this test stage all four channels are measured. Measure the channels sequentially as they cannot be measured at the same time.

For loopback delay detection, use the following test procedure for either BTL mode or PBTL mode:

• BTL mode
  1. Set the AC_DIAGS_LOOPBACK bit (bit 7 in register 0x16) to '0' to disable AC loopback mode.
  2. Apply a 0-dBFS 19 kHz signal and enable AC load diagnostics. CH1 and CH2 reuse the AC sensing loop of CH1 (set bit 3 in register 0x15 to '1'). CH3, CH4 reuse the AC sensing loop of CH3 (set bit 1 in register 0x15 to '1').
  3. Read back the 16-bit hexadecimal, AC_LDG_PHASE1 value. Register 0x1B holds the MSB and register 0x1C holds the LSB.
  4. Read back the hexadecimal stimulus value, STI. Register 0x1D holds the MSB and register 0x1E holds the LSB.
  5. For channel 1/2 set bit 3 in register 0x15 to '0'. For channel 3/4 set bit 1 in register 0x15 to '0'.

     When the test is complete, the channel reporting register indicates the status change from the AC diagnostic mode to the Hi-Z state. The detected impedance is stored in the appropriate I²C register.

• PBTL mode
  1. Set the AC_DIAGS_LOOPBACK bit (bit 7 in register 0x16) to '0' to disable AC loopback mode.
  2. Set the PBTL CH12 and PBTL CH34 bits (see register 0x00) to '0' without toggling STANDBY pin to enter BTL mode only for load diagnostics.
  3. Apply a 0 dBFS 19 kHz signal and enable AC load diagnostics. For PBTL12, enable the AC sensing loop of CH1 (set bit 3 in register 0x15 to '1'). For PBTL34, enable the AC sensing loop of CH3 (set bit 1 in register 0x15 to 1).
  4. Read back the AC_LDG_PHASE1 value. Register 0x1B holds the MSB and register 0x1C holds the LSB.
  5. Read back the hexadecimal stimulus value, STI. Register 0x1D holds the MSB and register 0x1E holds the LSB.
  6. Set the PBTL CH12 and PBTL CH34 bits (see register 0x00) to '1' to go back to PBTL mode for load diagnostics.
  7. For PBTL12 set bit 3 in register 0x15 to '0'. For PBTL34 set bit 1 in register 0x15 to '0'.

The AC phase in degrees is calculated with the Equation 2.

Equation 2.

Where:

• Phase_CHx(LBK) is the reference phase measurement. LBK stands for loopback mode
• Phase_CHx(LDM) is the phase measure of the load. LDM stands for load mode
• STI_CHx(LDM) is the stimulus value