Resistive touch screen wrong coordinates

Hi.

I have a problem withresistive TP.

I use STMPE811.

I use my custom board with connection of the stmpe811 similar to the stm32f429discovery board, as well as the drivers from the board demonstration software.

I cannot get correctvalues of x and y.

To isolate the problem I take the values of x and y immediately after this function –stmpe811_TS_GetXY(...) and print them to the screen. The values are quite unstable. For example when I push in some part of the screen I receive something like this x = 1230, y = 3457. Holding the finger in the same spot the values of x and y change slightly (+-50) but once in 2-3 seconds those values are something absolutely different like x= 60, y = 6354234.

Another problem is when I move finger along the x axis the value of y also changes by several times (but it should stay the same as I don't change y position).

I tried to change touch detect delay, average control samples – the problem had notchanged.

Just for reference hereare the values of the corners:

1. left upper x = 2720, y = 3500

2. left lower x = 1900, y = 4300

3. right upper x = 4020, y = 3600

4. right lower x = 530, y = 52014

5. middle of the screen x = 3670, y = 2180

As I mentioned the values are something absoultely different once per 2-3 seconds.

Thanks.