參數(shù)資料
型號(hào)： EVAL-AD7877EB
廠商： Analog Devices, Inc.
英文描述： Touch Screen Controller
中文描述： 觸摸屏控制器
文件頁(yè)數(shù)： 25/44頁(yè)
文件大小： 1002K
代理商： EVAL-AD7877EB
AD7877
If
f
DCLK
= 20 MHz (maximum), then
t
DCLK
= 50 ns.
Rev. A | Page 25 of 44
t
2
=
timer interval
×
t
DCLK
= (1024 × 50 ns) = 51.2 μs
T
WRITE
=
T
READ
= 16 clk period ×
t
DCLK
= 800 ns
t
3
= maximum time taken to write read address and read
11 registers = 800 ns (write) + [800 ns (read) × 11] = 9.6 μs.
t
4MAX
=
t
2
t
3
= 51.2 μs 9.6 μs = 41.6 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 42. This is a digital logic output
with an internal pull-up resistor of 50 k, which means it does
not need an external pull-up. The PENIRQ output idles high.
The PENIRQ circuitry is always enabled, except during
conversions.
0
X+
TOUCH
SCREEN
Y+
50k
Y–
X–
PENIRQ
ENABLE
PENIRQ
V
CC
V
CC
Figure 42. PENIRQ Output Equivalent Circuit
When the screen is touched, PENIRQ goes low. This can be
used to generate an interrupt request to the host. When the
screen touch ends, PENIRQ goes high immediately, if the ADC
is idle. If the ADC is converting, PENIRQ goes high when the
ADC becomes idle. The PENIRQ operation for these two
conditions is shown in Figure 43.
0
SCREEN
PENIRQ
ADC
STATUS
TOUCHED
NOT
TOUCHED
NOT
TOUCHED
NOT
TOUCHED
NOT
TOUCHED
ADC IDLE
SCREEN
PENIRQ
ADC
STATUS
TOUCHED
ADC IDLE
ADC
CONVERTING
ADC IDLE
RELEASE NOT
DETECTED
PENIRQ
RELEASE
PENIRQ
RELEASE
PENIRQ
DETECTS
PENIRQ
DETECTS
Figure 43. PENIRQ Operation for ADC Idle and ADC Converting
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 10b, and can
read the result registers after the conversions have 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, so, when the screen is touched, the programmed
sequence of conversions begins automatically. When the DAV
signal asserts, the host reads the new data available in the
AD7877 results registers and returns to sleep mode. This
method can significantly reduce the load on the host.
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