Programming Neo FreeRunner LEDs

Neo FreeRunner has three LED indicators:

Color	Location	IO Port/pin
orange	behind power button	GPB0
blue	behind power button	GPB1
red	behind aux button	GPB2

Combination of orange and blue behind the power button should give purple.

Port B Control Registers

Register	Address	Description	Reset value
GPBCON	0x56000010	Pin configuration	0x0
GPBDAT	0x56000014	Data in/out	undefined
GPBDN	0x56000018	Pull-down disable	0x0

GPB has 11 pins (GPB0 through GPB11 inclusive).

Register descriptions:

• GPBCON: Two configuration bits per pin (GPBCON[1:0] for GPB0, GPBCON[3:2] for GPB1, etc.), 00 = Input, 01 = Output, 10 = XXX, 11 = reserved. XXX refers to internal signals, different for each pin.
• GPBDAT[10:0]: Write GPB10..GPB0 in output mode and read GPB10..GPB0 in input mode.
• GPBDN[10:0]: 0 = pull-down enabled, 1 = pull-down disabled in GPB10..GPB0.

For blinking LEDs, we select output mode (0b01) for GPB0..GPB2. Pull-down should be disabled. Then write to GPBDAT[0:2]. To turn the LED on, set the pin value to 1 (H level), to turn it off set pin value to 0 (L level).

References:

• ​Samsung S3C2442B Processor Datasheet Port B Control Registers, Page 9-10.
• ​Neo FreeRunner Schematics MCU, sheet 3.

Notes:

• Could not find in datasheet whether H drive is coded as 1 or 0. From experiments 1 is H because 1 lights the LED.

I/O registers vs. endianity

All the I/O registers on the S3C24xx are naturally 32-bit. This means if you access them as 32-bit memory locations, you needn't worry about endianity (recommended). If you need to access them as 8-bit memory locations, the GTA02 memory system is little-endinan. You would thus use the same address for the 8-bit access as for the 32-bit access in order to read or write the least significant byte.