AD7303
–14–
REV. 0
AD7303 to 68HC11 Interface Program Source Code
*
PORTC EQU $1003 Port C Control Register
* "SYNC, 0, 0, 0, 0, 0, 0, 0"
DDRC EQU $1007 Port C Data Direction
PORTD EQU $1008 Port D Data Register
* "0, 0, 0, SCLK, DIN, 0, 0, 0"
DDRD EQU $1009 Port D Data Direction
SPCR EQU $1028 SPI Control Register
* "SPIE, SPE, DWOM, MSTR, CPOL, CPHA, SPR1, SPR0"
SPSR EQU $1029 SPI Status Register
* "SPIF, WCOL, 0, MODF, 0, 0, 0, 0"
SPDR EQU $102A SPI Data Register, Read Buffer, Write Shifter
*
* SDI RAM Variables: DIN 1 is eight MSBs, Control BYTE
DIN 2 is eight LSBs, Data BYTE
DAC requires 2*8-bit Writes
DIN1 EQU $00 DIN BYTE 1: "
INT/EXT, X, LDAC, PDB, PBA, A/B, CR1, CR0"
DIN2 EQU $01 DIN BYTE 2: " DB7, DB6, DB5, DB4, DB3, DB2, DB1, DB0"
*
ORG $C000 Start of users ram
INIT LDS #$CFFF Top of C page Ram
*
LDAA #$80 1, 0, 0, 0, 0, 0, 0, 0
* SYNC is High
STAA PORTC Initialize Port C Outputs
LDAA #$80 1, 0, 0, 0, 0, 0, 0, 0
STAA DDRC SYNC enabled as output
*
LDAA #$00 0, 0, 0, 0, 0, 0, 0, 0
* SCLK is low, DIN is low
STAA PORTD Initialize Port D outputs
Power Supply Bypassing and Grounding
In any circuit where accuracy is important, careful consideration
of the power supply and ground return layout helps to ensure
the rated performance. The printed circuit board on which the
AD7303 is mounted should be designed so that the analog and
digital sections are separated, and confined to certain areas of
the board. If the AD7303 is in a system where multiple
devices require an AGND to DGND connection, the connec-
tion should be made at one point only. The star ground point
should be established as closely as possible to the AD7303. The
AD7303 should have ample supply bypassing of 10 µF in paral-
lel with 0.1 µF on the supply located as closely to the package as
possible, ideally right up against the device. The 10 µF capaci-
tors are the tantalum bead type. The 0.1 µF capacitor should
have low Effective Series Resistance (ESR) and Effective Series
Inductance (ESI), like the common ceramic types that provide a
low impedance path to ground at high frequencies to handle
transient currents due to internal logic switching.
The power supply lines of the AD7303 should use as large a
trace as possible to provide low impedance paths and reduce the
effects of glitches on the power supply line. Fast switching sig-
nals such as clocks should be shielded with digital ground to
avoid radiating noise to other parts of the board, and should
never be run near the reference inputs. Avoid crossover of digi-
tal and analog signals. Traces on opposite sides of the board
should run at right angles to each other. This reduces the effects of
feedthrough through the board. A microstrip technique is by far
the best, but not always possible with a double-sided board. In
this technique, the component side of the board is dedicated to
ground plane while signal traces are placed on the solder side.
