MCP3004_07 datasheet(17/34 Pages) MICROCHIP

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DS21295C-page 17

MCP3004/3008

6.0

APPLICATIONS INFORMATION

6.1

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.

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