AD660–Microprocessor Interface Section
AD660 TO MC68HC11 (SPI BUS) INTERFACE
The AD660 interface to the Motorola SPI (serial peripheral in-
terface) is shown in Figure 8. The MOSI, SCK, and
SS
pins of
the HC11 are respectively connected to the BIT0,
CS
and
LDAC pins of the AD660. The
SER
pin of the AD660 is tied
low causing the first rank latch to be transparent. The majority
of the interfacing issues are taken care of in the software initial-
ization. A typical routine such as the one shown below begins by
initializing the state of the various SPI data and control registers.
The most significant data byte (MSBY) is then retrieved from
memory and processed by the SENDAT subroutine. The
SS
pin is driven low by indexing into the PORTD data register and
clear Bit 5. This causes the 2nd rank latch of the AD660 to be-
come transparent. The MSBY is then set to the SPI data regis-
ter where it is automatically transferred to the AD660.
The HC11 generates the requisite 8 clock pulses with data valid
on the rising edges. After the most significant byte is transmit-
ted, the least significant byte (LSBY) is loaded from memory
and transmitted in a similar fashion. To complete the transfer,
the LDAC pin is driven high latching the complete 16-bit word
into the AD660.
INIT
LDAA
STAA
LDAA
STAA
LDAA
STAA
LDAA
BSR
JMP
LDY
BCLR
STAA
LDAA
BPL
LDAA
STAA
LDAA
BPL
BSET
RTS
#$2F
PORTD
#$38
DDRD
#$50
SPCR
MSBY
SENDAT
NEXTPT
#$1000
$08,Y,$20
SPDR
SPSR
WAIT1
LSBY
SPDR
SPSR
WAIT2
$08,Y,$20
;SS = I; SCK = 0; MOSI = I
;SEND TO SPI OUTPUTS
;SS, SCK,MOSI = OUTPUTS
;SEND DATA DIRECTION INFO
;DABL INTRPTS,SPI IS MASTER & ON
;CPOL=0, CPHA = 0,1MHZ BAUD RATE
;LOAD ACCUM W/UPPER 8 BITS
;JUMP TO DAC OUTPUT ROUTINE
;INFINITE LOOP
;POINT AT ON-CHIP REGISTERS
;DRIVE SS (LDAC) LOW
;SEND MS-BYTE TO SPI DATA REG
;CHECK STATUS OF SPIE
;POLL FOR END OF X-MISSION
;GET LOW 8 BITS FROM MEMORY
;SEND LS-BYTE TO SPI DATA REG
;CHECK STATUS OF SPIE
;POLL FOR END OF X-MISSION
;DRIV
SS
HIGH TO LATCH DATA
MICROWIRE
™
SO
SK
G1
BIT0
CS
LDAC
SER
AD660
Figure 9. AD660 to MICROWIRE Interface
AD660 TO ADSP-210x FAMILY INTERFACE
The serial mode of the AD660 minimizes the number of control
and data lines required to interface to digital signal processors
(DSPs) such as the ADSP-210x family. The application in Fig-
ure 10 shows the interface between an ADSP-2101 and the
AD660. Both the TFS pin and the DT pins of the ADSP-2101
should be connected to the
SER
and BIT0 pins of the AD660,
respectively. An inverter is required between the SCLK output
and the
CS
input of the AD660 in order to assure that data
transmitted to the BIT0 pin is valid on the rising edge of
CS.
The serial port (SPORT) of the DSP should be configured for
alternate framing mode so that TFS complies with the word-
length framing requirement of
SER.
Note that the INVTFS bit
in the SPORT control register should be set to invert the TFS
signal so that
SER
is the correct polarity. The LDAC signal,
which must meet the minimum hold specification of t
IH
, is easily
generated by delaying the rising edge of
SER
with a 74HC74
flip-flop. The
CS
signal clocks the flip-flop resulting in a delay
of approximately one
CS
clock cycle.
In applications such as waveform generation, accurate timing of
the output samples is important to avoid noise that would be in-
duced by jitter on the LDAC signal. In this example, the
ADSP-2101 is set up to use the internal timer to interrupt the
processor at the precise and desired sample rate. When the
timer interrupt occurs, the processors’s 16-bit data word is writ-
ten to the transmit register (TXn). This causes the DSP to auto-
matically generate the TFS signal and begin transmission of the
data.
ADSP-210x
SCLK
74HC04
CS
BIT0
NEXTPT
SENDAT
WAIT1
WAIT2
68HC11
MDSI
SCK
SS
BIT0
CS
LDAC
SER
DT
AD660
AD660
TFS
D
Q
74HC74
LDAC
SER
Figure 8. AD660 to 68HC11 (SPI) Interface
AD660 TO MICROWIRE INTERFACE
Figure 10. AD660 to ADSP-210x Interface
AD660 TO Z80 INTERFACE
The flexible serial interface of the AD660 is also compatible
with the National Semiconductor MICROWIRE™ interface.
The MICROWIRE interface is used on microcontrollers such as
the COP400 and COP800 series of processors. A generic inter-
face to the MICROWIRE interface is shown in Figure 9. The
G1, SK, And SO pins of the MICROWIRE interface are respec-
tively connected to the LDAC,
CS
and BIT0 pins of the
AD660.
MICROWIRE is a registered trademark of National Semiconductor.
Figure 11 shows a Zilog Z-80 8-bit microprocessor connected to
the AD660 using the byte mode interface. The double-buffered
capability of the AD660 allows the microprocessor to indepen-
dently write to the low and high byte registers, and update the
DAC output. Processor speeds up to 6 MHz on Z-80B require
no extra wait states to interface with the AD660 using a
74ALS138 as the address decoder.
–10–
REV. A
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