It's a TI TSC2046 controller, hooked up to the ARM7 SPI port. News at 9.

Command Format:

bit	name	description
7	S	Start bit
6..4	A2..A0	Channel select
3	Mode	12 bit/8 bit conversion mode
2	SER/DFR	read type
1..0	PD1..PD0	power-down mode

Power down mode:

PD1	PD0	/PENIRQ	Description
0	0	Enabled	Power-down between conversions
0	1	Disabled	Voltage reference off, ADC on
1	0	Enabled	Voltage reference on, ADC off
1	1	Disabled	Device always powered. Voltage reference on, ADC on

/PENIRQ is wired to bit 6 of R2_CR

The DS has wired an external VREF of 3.3 V to the TSC, instead of using the internal 2.5 VREF. This impacts the temperature calculations below, and converting any reading into an actual voltage.

Useful commands:

Constant	Value	Notes
TSC_MEASURE_TEMP1	0x84	Measures temperature diode 1
TSC_MEASURE_Y	0x94	Measures Y position
TSC_MEASURE_BATTERY	0xA4	Does not work on DS, VBAT is grounded
TSC_MEASURE_Z1	0xB4	Measures cross-panel position 1
TSC_MEASURE_Z2	0xC4	Measures cross-panel position 2
TSC_MEASURE_X	0xD4	Measures X position
TSC_MEASURE_AUX	0xE4	Measures ?, its non-zero, but I don't know what
TSC_MEASURE_TEMP2	0xF4	Measures temperature diode 2

A 12-bit measurment cycle consists of 3 SPI byte transfers:

• The first transfer sends the command and returns no meaningful data.
• The second transfer returns 0 measurement(11)..measurment(5)
• The third transfer returns measurment(4..0) 000

Thus, pseudocode for measuring a TSC input is:

   BlockingWriteSPI(command);  data = BlockingWriteSPI(0);  data = (data << 5) | (BlockingWriteSPI(0) >> 3);    

Temperature calculation:

Temperature diode two has a 91 times larger current, and the absolute temperature can be calculated based on the measurements of the two sensors. I'll present a fixed point algorithm for computing the temperature in degrees C (see the TSC 2046 datasheet for the original equation).

Reading from the touchscreen controller (SPI)

uint16 touchRead(uint32 command) {
uint16 result;
while (SERIAL_CR & SERIAL_BUSY) swiDelay(1);

// Write the command and wait for it to complete
SERIAL_CR = SERIAL_ENABLE | 0x800 | 0x201;
SERIAL_DATA = command;
while (SERIAL_CR & SERIAL_BUSY) swiDelay(1);

// Write the second command and clock in part of the data
SERIAL_DATA = 0;
while (SERIAL_CR & SERIAL_BUSY) swiDelay(1);
result = SERIAL_DATA;

// Clock in the rest of the data (last transfer)
SERIAL_CR = SERIAL_ENABLE | 0x201;
SERIAL_DATA = 0;
while (SERIAL_CR & SERIAL_BUSY) swiDelay(1);

// Return the result
return ((result & 0x7F) << 5) | (SERIAL_DATA >> 3);
}

  To do: figure out what bit 11, bit 9, and bit 0 are doing (bit 0   is probably the CR select, since the firmware reads have this bit cleared) 

Position calculation

#define SCREEN_WIDTH 256
#define SCREEN_HEIGHT 192

// those are pixel positions of the two points you click when calibrating
#define TOUCH_CNTRL_X1 (*(vu8*)0x027FFCDC)
#define TOUCH_CNTRL_Y1 (*(vu8*)0x027FFCDD)
#define TOUCH_CNTRL_X2 (*(vu8*)0x027FFCE2)
#define TOUCH_CNTRL_Y2 (*(vu8*)0x027FFCE3)

// those are the corresponding touchscreen values:
#define TOUCH_CAL_X1 (*(vu16*)0x027FFCD8)
#define TOUCH_CAL_Y1 (*(vu16*)0x027FFCDA)
#define TOUCH_CAL_X2 (*(vu16*)0x027FFCDE)
#define TOUCH_CAL_Y2 (*(vu16*)0x027FFCE0)

// linear mapping can be used to go from touchscreen position to pixel position

// precalculate some values
static int16 TOUCH_WIDTH = TOUCH_CAL_X2 - TOUCH_CAL_X1;
static int16 TOUCH_HEIGHT = TOUCH_CAL_Y2 - TOUCH_CAL_Y1;
static int16 CNTRL_WIDTH = TOUCH_CNTRL_X2 - TOUCH_CNTRL_X1;
static int16 CNTRL_HEIGHT = TOUCH_CNTRL_Y2 - TOUCH_CNTRL_Y1;

// reading pixel position:
int16 x = (IPC->touchX - (int16) TOUCH_CAL_X1) * CNTRL_WIDTH / TOUCH_WIDTH + (int16) TOUCH_CNTRL_X1;
int16 y = (IPC->touchY - (int16) TOUCH_CAL_Y1) * CNTRL_HEIGHT / TOUCH_HEIGHT + (int16) TOUCH_CNTRL_Y1;