PON controllers integrate a 13-bit Bias DAC, which operates in either manual or automatic mode. This application note describes the procedure for programming the DAC in the manual mode.
Controlling the BIAS DAC
If the BIAS-EN bit (Table 02h, Register 80h) is written to 0, the BIAS DAC is manually controlled by the MAN IBIAS Register (Table 02h, Registers F8h–F9h).
The MAN IBIAS Register is a 14-bit register, configured as described below.
MAN IBIAS Register
Programming in the Manual Mode
To program the BIAS DAC Register, the user must have Password Level 2 access. Also the BIAS-EN bit should be set to 0. To change the value, the MAN_CNTL Register (Table 02h, Registers FAh) is used to clock the MAN IBIAS value to register into the Bias DAC. The procedure for writing the DAC is then:
- Write the MAN IBIAS value with a write command.
- Set the MAN_CLK bit to a 1 with a separate write command.
- Clear the MAN_CLK bit to a 0 with a separate write command.
Programming a 13-Bit DAC with a 14-Bit Register
The MAN IBIAS Register is 14 bits, however, the Bias DAC is only 13 bits in resolution. The bit 27
(bit 7 of register F9h and bit 0 of register F8h) in the MAN IBIAS Register is redundant. The rollover occurs in the following manner:
- Increasing DAC value from 0000h to 3FFFh
The lower 8 bits of the DAC are incremented using register F9h. When the register value reaches FFh, register F8h is incremented by 1 bit, and register F9h is set to 80h and starts incrementing. The process repeats until all bits are 1.
- Decreasing DAC value from 3FFFh to 0000h
Decrement the lower 8 bits of the DAC by using register F9h. When the register value reaches 00h, register F8h is decremented by 1 bit, and register F9h is set to 7Fh and starts decrementing. The process repeats until all bits in both registers are at 00h.
The programming procedure presented here controls the Bias DACs in the DS1863/DS1865 PON controllers.