Datasheet

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SLAS370C − APRIL 2002 − REVISED OCTOBER 2004
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38
APPLICATION INFORMATION
MICROCONTROLLER CONNECTIONS
The ADS7871 is quite flexible in interfacing to various microcontrollers. Connections using the hardware mode
of two types of controllers (Motorola M68HC11, Intel 80C51) are described below.
Motorola M68HC11 (SPI)
The Motorola M68HC11 has a three-wire (four if you count the slave select) serial interface that is commonly
referred to as SPI (serial peripheral interface), where the data is transmitted MSB first. This interface is usually
described as the microcontroller and the peripheral each having two 8-bit shift registers (one for receiving and
one for transmitting).
The transmit shift register of the microcontroller and the receive shift register of the peripheral are connected
together and vice versa. SCK controls the shift registers. SPI is capable of full duplex operation (simultaneous
read and write). The ADS7871 does not support full duplex operation. The ADS7871 can only be written to
or read from. It cannot do both simultaneously.
Since the M68HC11 can configure SCK to have either rising or falling edge active, the RISE/FALL pin on the
ADS7871 can be in whichever state is appropriate for the desired mode of operation of the M68HC11 for
compatibility with other peripherals.
In the Interface Control register (see Figure 32), the 2W/3W bit should be cleared (default). The LSB bit should
be clear (default). The 8051 bit should also be clear (default). Since the ADS7871 defaults to SPI mode, the
M68HC11 should not need to initialize the ADS7871 Interface Configuration register after power-on or reset.
Figure 44 shows a typical physical connection between an M68HC11 and a ADS7871. A pull-up resister on
DOUT may be needed to keep DOUT from floating during write operations. CS may be permanently tied low
if desired, but then the ADS7871 must be the only peripheral.
M68HC11 ADS7871
MISO
MOSI
SCK
SS
DIN
DOUT
SCLK
CS
10 kΩ typ
V
DD
Figure 44. Connection of a M68HC11 to an ADS7871