AD7877 Analog Devices, AD7877 Datasheet - Page 25

• Manufacturer Part Number: AD7877
• Description: Touch Screen Controller
• Manufacturer: Analog Devices

Specifications of AD7877

• Resolution (bits): 12bit
• # Chan: 9
• Sample Rate: 125kSPS
• Interface: Ser
• Analog Input Type: SE-Uni
• Ain Range: Uni (Vref),Uni 2.5V
• Adc Architecture: SAR
• Pkg Type: CSP

Data Sheet
If f
t
11 registers = 800 ns (write) + [800 ns (read) × 11] = 9.6 µs.
Pen Interrupt ( PENIRQ )
The pen interrupt request output ( PENIRQ ) goes low whenever
the screen is touched. The pen interrupt equivalent output
circuitry is outlined in Figure 43. This is a digital logic output
with an internal pull-up resistor of 50 kΩ, which means it does
not need an external pull-up. The PENIRQ output idles high.
The PENIRQ circuitry is always enabled, except during
conversions.
When the screen is touched, PENIRQ goes low. This generates
an interrupt request to the host. When the screen touch ends,
and if the ADC is idle, PENIRQ immediately goes high. If the
ADC is converting, PENIRQ goes high once the ADC becomes
idle. The PENIRQ operation for these two conditions is shown
in Figure 44.
3
= maximum time taken to write read address and read
DCLK
X–
t
t
t
2
WRITE
4MAX
= 512 µs with timer set to 1024 (TMR bits = 01b)
= 20 MHz (maximum), then t
SCREEN
TOUCH
= t
= t
2
READ
Figure 43. PENIRQ Output Equivalent Circuit
− t
3
= 16 CLK period × t
= 512 µs − 9.6 µs = 502.4 µs
X+
Y+
Y–
ENABLE
PENIRQ
V
CC
DCLK
DCLK
50kΩ
= 50 ns.
= 800 ns
V
CC
PENIRQ
Rev. D | Page 25 of 44
SCREEN
SCREEN
SYNCRONIZING THE AD7877 TO THE HOST CPU
The two suggested methods for synchronizing the AD7877
to its host CPU are slave mode, in which the mode bits can
be either 01b or 10b, and master mode, in which the mode
bits are 11b.
In slave mode, PENIRQ can be used as an interrupt to the host.
When PENIRQ goes low to indicate that the screen has been
touched, the host is awakened. The host can then program the
AD7877 to begin converting in either Mode 01b or Mode 10b,
and can read the result registers after the conversions are
completed.
In master mode, DAV can also be used as an interrupt to the
host. However, the host should first initialize the AD7877 in
Mode 11b. The host can then go into sleep mode to conserve
power. The wake-up on touch feature of the AD7877 is active in
this mode, therefore, when the screen is touched, the
programmed sequence of conversions begins automatically.
When the DAV signal asserts, the host reads the new data avail-
able in the AD7877 results registers and returns to sleep mode.
This method can significantly reduce the load on the host.
STATUS
STATUS
PENIRQ
PENIRQ
ADC
ADC
Figure 44. PENIRQ Operation for ADC Idle and ADC Converting
TOUCHED
TOUCHED
NOT
NOT
ADC IDLE
PENIRQ
DETECTS
TOUCH
PENIRQ
DETECTS
TOUCH
TOUCHED
TOUCHED
ADC IDLE
CONVERTING
ADC
RELEASE NOT
DETECTED
PENIRQ
DETECTS
RELEASE
ADC IDLE
PENIRQ
DETECTS
RELEASE
AD7877
TOUCHED
TOUCHED
NOT
NOT