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MK715

The MK715 communicates via a 4 pin serial port. This may be connected as either a 3 or 4 wire

serial port. The port is connected to 4 registers that control the various modes and function of the

chip.

The primary function of the MK715 is to control resistive touchscreens. There are two ways to read

screen points, both controlled by the registers.

In the first method, which is enabled by setting the ENCONR bit, the MK715 performs periodic

conversions at a rate set by the rate register. The chip monitors the touch screen in a low power

condition (about 4 mA) until the screen is touched. When a touch is detected, the chip powers up

and starts converting screen points. The TOUCH# pin goes low and INT goes high to indicate a

change in touch status. The converter outputs a Y co-ordinate, then an X co-ordinate, then a Y co-

ordinate, and so on. The X and Y co-ordinates are stored in the same register (RESULT) and each

conversion over-writes the previous conversion. When a co-ordinate is stored, the conversion

complete bit is asserted in the STATUS register. This bit is cleared when the RESULT register is

read. The inverted state of the TOUCH# pin also appears in the STATUS register. After each co-

ordinate conversion, INT goes high and the screen is checked to see if it is still touched. If not,

conversions stop, TOUCH# goes high, INT goes high (to indicate a change in touch status) and

the chip reverts to the low power mode.

The second method to read a screen is to set the RD1PT (read one point) bit in the CONTROL

register. The chip will perform two conversions, a Y co-ordinate followed by an X co-ordinate. The

X co-ordinate overwrites the Y co-ordinate and so the X co-ordinate must be read before this

happens. Finally, RD1PT is cleared. The conversion pair takes about 3.5 ms.

The converter may also be used to measure voltages presented on the GP or PL pins. The range

of the converter is 0 to 1.279 V and so voltages outside this range must be scaled appropriately.

Again, the RD1PT bit is set to start the conversion but first either SELGP or SELPL must be set

to select the correct input. Only one conversion is performed. The result is stored in the RESULT

register and then RD1PT is cleared. The conversion takes about 1.7ms.

The final conversion mode is used to measure touch pressure. This is controlled identically to the

second method outlined above except that either RDPRESA or RDPRESB must first be set.

The MK715 allows for several different clocks to be generated, controlled by the registers. On the

CLKOUT pin, the output is either a a 32768 Hz clock from the crystal oscillator or a MHz - frequency

clock synthesized from the PLL. Similarly, this MHz - frequency clock can appear on the INT or

TOUCH# pins instead of their usual functions. In these cases, if the MHz clock needs to run

continuously, then the ENPLLbit must be set in order to override the automatic powerdown of the PLL.

Refer to page 12 for more details.

Chip Overview