MCP3004/3008
6.0
6.1
APPLICATIONS INFORMATION
Using the MCP3004/3008 with
Microcontroller (MCU) SPI Ports
With most microcontroller SPI ports, it is required to
send groups of eight bits. It is also required that the
microcontroller SPI port be configured to clock out data
on the falling edge of clock and latch data in on the ris-
ing edge. Because communication with the MCP3004/
3008 devices may not need multiples of eight clocks, it
will be necessary to provide more clocks than are
required. This is usually done by sending ‘leading
zeros’ before the start bit. As an example, Figure 6-1
and Figure 6-2 shows how the MCP3004/3008 can be
interfaced to a MCU with a hardware SPI port.
Figure 6-1 depicts the operation shown in SPI Mode
0,0, which requires that the SCLK from the MCU idles
in the ‘low’ state, while Figure 6-2 shows the similar
case of SPI Mode 1,1, where the clock idles in the ‘high’
state.
As is shown in Figure 6-1, the first byte transmitted to
the A/D converter contains seven leading zeros before
the start bit. Arranging the leading zeros this way
induces the 10 data bits to fall in positions easily manip-
ulated by the MCU. The MSB is clocked out of the A/D
converter on the falling edge of clock number 14. Once
the second eight clocks have been sent to the device,
the MCU receive buffer will contain five unknown bits
(the output is at high impedance for the first two
clocks), the null bit and the highest order 2 bits of the
conversion. Once the third byte has been sent to the
device, the receive register will contain the lowest order
eight bits of the conversion results. Employing this
method ensures simpler manipulation of the converted
data.
Figure 6-2 shows the same thing in SPI Mode 1,1,
which requires that the clock idles in the high state. As
with mode 0,0, the A/D converter outputs data on the
falling edge of the clock and the MCU latches data from
the A/D converter in on the rising edge of the clock.
© 2007 Microchip Technology Inc.
DS21295C-page 17
MICROCHIP [ MICROCHIP ]
