readme.txt

C Sourcr cdoe 採用AD7714 電能表数据和计量脉冲输出

This package contains the code for a digital voltmeter based upon
an ATMEGA168 controller, an AD7714 22-bit ADC, and an LCD panel.
The AVR is clocked at 16MHz and the ADC is clocked at 2.4576MHz,
with sampling rates suitable for filtering out 60Hz noise.

The LCD panel is accessed via the SPI port by using the 74HC595
shift register chip.  The LCD is programmed in 4-bit mode.
Menus can be navigated using a couple of rotary switches.
Data values are also sent to the serial port which is set to 
115200 baud.

The AD7714 driver is capable of operating in either interrupt mode
(triggered by the DRDY pin going low) or in non-threaded mode, depending
on a definition in config.h.  The ADC is set to bipolar 24-bit mode.

A Ruby script is included for visualizing the measurements on a
series of strip charts.  There are six charts, each with a gain ten
times more than the previous.  The script requires the gtk2 and
serialport Ruby libraries.  It is probably a lot easier to get the
gtk2 libraries working on Linux rather than Windows.

Some notes regarding the AD7714:
* I was using about 6 inches of cable between the SPI port
  and the ADC chip.  Occasional communication errors would lock
  the chip up no matter how slow the SPI port was clocked.
  Application note AN-406 mentioned something about wire inductance
  and internal blocking diodes.  The solution is to add 100 ohm
  resistors in front of all the digital inputs on the ADC.
* There is an internal buffer which, when enabled, allows the use of
  "high impedance" loads.  However, the datasheet specifies a
  leakage current of 1nA which is enough to induce a 1ppm error
  for loads with impedance greater than 2k ohm even at a gain of 1.
  There are op-amps with leakage of less than 10fA which could
  help here.
* I am getting an offset of about 20 units after doing an
  internal calibration, even on the AIN6-AIN6 test input which
  should always read zero.  I don't know if this is supposed to
  be normal.

AD7714 pins       To
-----------       ---------
SCLK              AVR PB5 (SCK)
DOUT              AVR PB4 (MISO)
DIN               AVR PB3 (MOSI)
/CS               AVR PB2 (/SS) with an external pull-up resistor
                  (the pull-up ensures that the ADC is disabled during
				  programming of the AVR via SPI port)
/DRDY             AVR PD2 (INT0)
BUFFER            AVR PD3
POL               +5V
/SYNC,/RESET      GND
/STANDBY          GND
MCLK IN,OUT       2.4576 MHz XTAL
REF IN(+)         Precision 2.5V source (AD780)
REF IN(-)         Analog ground

74HC595 pins      To
------------      ---------
Shift Clock       AVR PB5 (SCK)
Serial In         AVR PB3 (MOSI)
Latch Clock       AVR PB1
QA                LCD RS
QB                LCD R/W
QC                LCD SEL
QD                LCD Backlight
QE                LCD D4
QF                LCD D5
QG                LCD D6
QH                LCD D7
/G                GND
/SCLR             +5V

Rotary Switch     To
-------------     ---------
Rotary 1 Pin A    AVR PD4
Rotary 1 Pin B    AVR PD5
Rotary 2 Pin A    AVR PD6
Rotary 2 Pin B    AVR PD7

Other AVR Pins
--------------
PB6, PB7 => 16MHz Ceramic Resonator
PD0, PD1 => Serial Port
PB0 is reserved for a third SPI device (eg. memory card)