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SS7646

8/21/2004 Rev.2.01

The Control Byte :

Table I and Table II show detailed information of the

control byte (on DIN). The control byte provides the

start operation, addressing, resolution, reference and

power down information to the SS7646.

Start Bit –

Initiate Start

The control byte starts with the first high bit on DIN.

The first bit must always be HIGH (1) to initiate the start

of the conversion. The SS7646 will ignore any

inputs on the DIN until the start bit is detected.

Addressing Bits –

Input Channel Selection

The next three bits on control byte (A2, A1, A0) select

the active input channel of the input multiplexer (see

Table III, and Figure 5), and touch screen drivers.

Mode Bits –

Resolution and Reference Configuration

The Mode bits (Mode1, Mode0) set the resolution of

the analog-to-digital (ADC) converter. With the Mode1

bit LOW (0) the following conversion will have 12 bits of

resolution. With the Mode1 bit HIGH (1) the following

conversion will have 8 bits of resolution. The MODE0

bit set the reference input of the ADC (see Reference

Input section and Figure 5). Table IV shows detailed

i

nformation of the Mode Bits.

PD0 and PD1 Bits -

Power Down Control

The last two bits (PD1, PD0) control the power-down

mode and pen interrupt request of the SS7646. If both

bits are HIGH (1), the ADC is always powered up and

pen interrupt will be disabled. If both bits are LOW (0),

the ADC enters a power-down mode between

conversions. The internal 2.5V voltage reference can

be turned ON or OFF independently of the ADC with

the PD1 bit. See Table V for more information.

A2 A1 A0

Measure

Input

Channel

X Switch Y Switch

0 0

X+

OFF

ON

1 0

Y+

ON

OFF

0 1

X+

X- ON

Y+ ON

1 0

Y-

X- ON

Y+ ON

0 1

0

Battery

OFF

OFF

1 1

0

Auxiliary

OFF

OFF

0 0

0

TEMP0

-

OFF

OFF

1 1

1

TEMP1

-

OFF

OFF

Table III. Input Channel Configuration

Operation of 16-Clocks

The typical operation of the SS7646 is 24-clocks (three

control bytes) per conversion. However the control bits

for the next conversion can be overlapped with current

conversion for a faster conversion. Figure 3 shows the

timing of 16-Clocks per conversion.

Operation of 15-Clocks

The fastest operation (15-clocks per conversion) of

SS7646 is shown on Figure 4. This operation will

not wo r k wi th t he s e r i a l i nte r f a c e of m o s t

microcontrollers and digital signal processors, as they

are not capable of providing 15 clocks cycles per serial

transfer.

MODE1 MODE0

Resolution

ADC Reference

0

1

12bits

0

0

12bits

*

1

1

8bits

1

0

8bits

*

Table IV. Resolution and Reference Configuration

* When measuring Y(001), X(101), Z1(011) and Z2(100) positions,

otherwise Invalid configuration

PD1 PD0 /PENIRQ DESCRIPTION

0

0

Enable The reference is switching OFF

and the ADC will powerdown

betweenconversions. Y- switch is on

while in power-down.

0

1

Enable The reference is switching OFF

and the ADC is ON permanently.

No power down between

conversions.

1

0

Enable The reference is ON and the

ADC will power down between

conversions.

1

1

Disable The reference and the ADC are

ON. No power down between

conversions, the ADC always

power up. The pen interrupt

functionality will be disabled.

Table V

. Power-Down and Pen Interrupt Selection