AK4184 Datasheet, PDF (18/36 Pages) Asahi Kasei Microsystems – TSC with Keypad Scanner and GPIO Expander

[AK4184]
■ Touch Screen Control Sequence
The timing of sampling and A/D conversion is shown in Figure8. The AK4184 is controlled through a standard SPI
serial interface (CSN, SCLK, DIN, and DOUT). BUSY and DOUT are in Hi-Z state when CSN = “H”. The AK4184
latches the4-bit control word serially via DIN on the rising edge of SCLK. DIN must be low until CSN is “H”. The S
bit must be “1”.
The sampling (Sampling1 or Sampling2) time for the A/D converter depends on the PD bit. The SAR-A/D conversion
is synchronized with SCLK. The A/D conversion occurs between the falling edge of the sixth SCLK and the 20^th
SCLK, followed by a 12-bit serial data output. Sampling time is determined by the PD bit.
If a series of the measurements are the same and the previous PD bit is “0”, sampling occurs between the falling edge
of the third SCLK and the sixth SCLK (Sampling2). When the previous PD bit is “1”, sampling occurs between the
falling edge of CSN to the falling edge of the sixth SCLK (Sampling1+Sampling2). This is a longer sampling time.
If the measurements switch to another axis, the measurement axis is determined by the current setting of the [A1, A0]
bits, irrespective of the PD bit setting. Even through the previous PD bit is “1”, the valid sampling time is Sampling2
as shown in Figure 8.
Sampling time depends on SCLK and the source impedance. When the A/D input voltage does not reach the final
voltage during Sampling2, continue to measure the same axis with the PD bit = “1” or use a slower SCLK in order to
increase the settling time.
The BUSY pin stays “H” from beginning of the A/D conversion to the beginning of MSV data output (20th falling
edge of SCLK). BUSY is “L” for the other period. The AK4184 outputs A/D data with MSB first via DOUT from the
rising edge of the 21st SCLK.
CSN
SCLK
1 2 3 4 5 6 7 8 9 14 16 17 18 19 20 21 22 23 24 25 29 30 31 32
DIN
S A1 A0 PD
Control Command
Hi-Z
DOUT
Touch Screen Driver SW
Previous PD bit
Sampling2
PD = “0”
PD = “1”
Sampling1 + Sampling2
A/D conversion
0
Hi-Z
D11 D10 D9 D8 D4 D3 D2 D1 D0
A/D Data
Current PD bit
PD = “0”
PD = “1”
PD = “1”
PD = “0”
Figure 8. Touch Screen Operation
MS0603-E-00
18
2007/04