AD7877 Analog Devices, AD7877 Datasheet - Page 28

• 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

AD7877
SERIAL INTERFACE
The AD7877 is controlled via a 3-wire serial peripheral
interface (SPI). The SPI has a data input pin (DIN) for inputting
data to the device, a data output pin (DOUT) for reading data
back from the device, and a data clock pin (DCLK) for clocking
data into and out of the device. A chip-select pin ( CS ) enables
or disables the serial interface.
WRITING DATA
Data is written to the AD7877 in 16-bit words. The first four
bits of the word are the register address that directs the AD7877
to the register to write to. The next 12 bits are data. How the
AD7877 handles the data bits depends on the register address.
Register Address 0000b is a dummy address, which does
nothing. Register Addresses 0010b to Register Address 1110b
are 12-bit registers that perform various functions as described
in the register map.
DOUT
DCLK
WADD3
DIN
4-BIT REGISTER WRITE ADDRESS
CONTROL REGISTER 1 ADDRESS
D15
CS
1
0
1
2
EXTENDED WRITE ADDRESS
NOTES:
1
2
3
4
HIGH-Z
DATA IS CLOCKED OUT ON THE FALLING EDGE OF DCLK.
INPUT DATA IS SAMPLED ON THE RISING EDGE OF DCLK.
FOR 8-BIT REGISTERS, 8 LEADING ZEROS PRECEDE 8 BITS OF DATA.
REGISTER READ ADDRESS INCREMENTS AUTOMATICALLY, PROVIDED THAT A NEW ADDRESS IS NOT WRITTEN TO CONTROL REGISTER 1.
WADD2
D14
1
0
4-BIT ADDRESS + 12-BIT DATA
D15
D15
0000 + 12-BIT DATA
1
WADD1
D13
1
0
WRITING TO CONTROL REGISTER 1 TO SET ADC CHANNEL, MODE, AND READ REGISTER ADDRESS
RATIOMETRIC (DIFFERENTIAL)
WADD0
NORMAL (SINGLE-ENDED)/
D12
1
0
3
CONVERSION
SER/DFR CHADD3 CHADD2 CHADD1 CHADD0
EADD3
D11
D11
Figure 46. Designation of Data-Word Bits in AD7877 Write Operations
4-BIT EXTENDED ADDRESS
REGISTER n DATA
EADD2
D10
D10
EXTENDED WRITE OPERATION TO GPIO REGISTERS
ADC CHANNEL ADDRESS
EADD1
Figure 47. Overall Read/Write Timing
D9
D9
4
WRITING TO A REGISTER
Rev. D | Page 28 of 44
16-BIT DATA-WORD
EADD0
D8
D8
D0
16
D0
HIGH-Z
D7
D7
D7
Register Address 1111b is not a physical register, but enables an
extended writing mode that allows writing to the GPIO
configuration registers. When the register address is 1111b, the
next four bits of the data-word are the address of a GPIO
configuration register and the eight LSBs are the GPIO
configuration data. For details on the configuration of the GPIO
pins, see the General-Purpose I/O Pins section.
Register Address 0001b is a physical register, Control Register 1,
but this is a special register. It contains data for setting up the
ADC channel and operating mode, but Bit 20 to Bit 6 compose
the register address for reading. These define the register that is
read back during the next read operation. Control Register 1
should be the last register in the AD7877 to be programmed
before starting a conversion. The three types of data-words used
for writing are shown in Figure 46.
RADD4
12 BITS DATA
D15
D6
D6
D6
0000 + 12-BIT DATA
1
5-BIT READ REGISTER ADDRESS
RADD3
D5
D5
D5
REGISTER n + 1 DATA
8 BITS GPIO DATA
RADD2
3
D4
D4
D4
RADD1
D3
D3
D3
4
RADD0
D2
D2
D2
MODE 1 MODE 0
D1
D1
D1
OPERATING
MODE
Data Sheet
16
D0
D0
D0
D0