LNB Control

The SkyWalker-1 controls LNB power and band selection through dedicated GPIO pins on the FX2 microcontroller. No I2C transactions are involved in basic LNB control — voltage and tone are driven directly by port pins.

LNB Voltage (13V / 18V)

LNB voltage selects the polarization of the received signal. It is controlled via GPIO P0.4 using vendor command 0x8B (SET_LNB_VOLTAGE).

wValue	Voltage	GPIO P0.4	Polarization
0x00	13V	LOW	Vertical / Circular-Right
0x01	18V	HIGH	Horizontal / Circular-Left

The Linux kernel driver sets the voltage during the tuning sequence based on the requested polarization:

Kernel driver polarization mapping

// From gp8psk-fe.c set_voltage callback
case SEC_VOLTAGE_13:  // Vertical / Circular-R
    gp8psk_usb_out_op(d, SET_LNB_VOLTAGE, 0, 0, NULL, 0);
    break;
case SEC_VOLTAGE_18:  // Horizontal / Circular-L
    gp8psk_usb_out_op(d, SET_LNB_VOLTAGE, 1, 0, NULL, 0);
    break;

The configuration status byte (returned by GET_8PSK_CONFIG, command 0x80) reflects the current voltage selection in bit 5 (0x20, bmSEL18V).

Extra Volt Mode (+1V Boost)

For long cable runs where voltage drop is significant, the SkyWalker-1 supports a +1V boost mode (13V becomes 14V, 18V becomes 19V). This is controlled via vendor command 0x94 (USE_EXTRA_VOLT).

wValue	XRAM 0xE0B6	Result
0x00	0x62	Normal voltage (13V / 18V)
0x01	0x6A	Boosted voltage (14V / 19V)

The difference between the two XRAM values is bit 3 (0x08), which enables the boost on the LNB power regulator circuitry.

Caution

The LNB power output has firm current limits. The maximum non-continuous draw is 750 mA (for short bursts of several minutes). For permanent continuous load, do not exceed 450 mA to avoid overheating the onboard regulator. Exceeding these limits risks thermal damage to the SkyWalker-1 board.

The kernel driver exposes extra volt mode through the enable_high_lnb_voltage callback in the DVB frontend properties.

22 kHz Tone Generation

The 22 kHz tone selects the high or low band on a universal LNB. It is controlled via GPIO P0.3 using vendor command 0x8C (SET_22KHZ_TONE).

wValue	State	GPIO P0.3	LNB Band
0x00	OFF	LOW	Low band (9.75 GHz LO on universal LNB)
0x01	ON	HIGH	High band (10.6 GHz LO on universal LNB)

Note

P0.3 does not generate the 22 kHz carrier directly. Instead, it gates an external 22 kHz oscillator circuit on the PCB. When P0.3 is HIGH, the oscillator output is coupled onto the coaxial cable’s DC power line alongside the 13V/18V LNB voltage. When P0.3 is LOW, the oscillator is disabled and no tone is present.

The configuration status byte tracks the tone state in bit 4 (0x10, bm22kHz).

LNB Power Supply Enable

Before any LNB control is possible, the LNB power supply itself must be enabled via vendor command 0x8A (START_INTERSIL). The kernel driver does this during the boot sequence if bit 2 (bmIntersilOn) of the configuration status byte is not already set.

wValue	Action	Config Bit
0x01	Enable LNB power supply	Sets bmIntersilOn (bit 2)
0x00	Disable LNB power supply	Clears bmIntersilOn (bit 2)

In the custom v3.01.0 firmware, enabling the LNB supply also configures the output enable masks for the LNB control GPIO pins:

START_INTERSIL handler in custom firmware

case START_INTERSIL:
    if (wval) {
        OEA |= (PIN_22KHZ | PIN_LNB_VOLT | PIN_DISEQC);
        config_status |= BM_INTERSIL;
    } else {
        config_status &= ~BM_INTERSIL;
    }

GPIO Pin Assignments

The LNB control pins are consistent across firmware versions, though their exact initialization differs:

GPIO Pin	Function	Direction	Init State
P0.1	Power supply enable	Output	LOW (off)
P0.2	Power supply disable	Output	HIGH (active disable)
P0.3	22 kHz tone gate	Output	LOW (tone off)
P0.4	LNB voltage select	Output	LOW (13V)
P0.5	BCM4500 hardware reset	Output	LOW (reset asserted)

The initial GPIO state after power-on is IOA = 0x84 (P0.7 and P0.2 HIGH, all others LOW), with the output enable mask OEA = 0xBE (P0.1 through P0.5 and P0.7 as outputs).

LNB GPIO Debug Commands

Three vendor commands provide low-level GPIO access for LNB debugging:

Command	Name	Purpose
0x96	SET_LNB_GPIO_MODE	Configure LNB GPIO output enables
0x97	SET_GPIO_PINS	Direct write to LNB GPIO pins via wValue bitmap
0x98	GET_GPIO_STATUS	Read LNB feedback GPIO pin (1 byte)

These commands access Port B (XRAM-mapped IOB) rather than Port A. In v2.06 and v2.13, 0x97 controls IOB bits 1 and 2 for LNB, while 0x96 configures IOB bit 3 as the GPIO mode select. Rev.2 uses IOB bit 4 instead.

Note

These debug GPIO commands are not used by the Linux kernel driver during normal operation. They are available for diagnostic tools and custom applications that need direct hardware-level LNB control.

Complete LNB Configuration Sequence

During a typical tune operation, the kernel driver configures the LNB in this order:

1. SET_LNB_VOLTAGE (0x8B)  -- Select 13V or 18V based on polarization
2. SET_22KHZ_TONE  (0x8C)  -- Select low or high band
3. SEND_DISEQC     (0x8D)  -- Switch multi-port switch if needed
4. TUNE_8PSK       (0x86)  -- Send tuning parameters

The voltage and tone are set before any DiSEqC commands because the LNB must be powered and in the correct band before the switch can respond. See the DiSEqC Protocol page for details on switch control, and the Tuning Protocol for the complete tune flow.