Datasheet

MCP6S21/2/6/8
DS21117B-page 18  2003-2012 Microchip Technology Inc.
5.0 DIGITAL FUNCTIONS
The MCP6S21/2/6/8 PGAs use a standard SPI com-
patible serial interface to receive instructions from a
controller. This interface is configured to allow daisy
chaining with other SPI devices. There is an internal
POR (Power On Reset) that resets the registers under
low power conditions.
5.1 SPI Timing
Chip Select (CS) toggles low to initiate communication
with these devices. The first byte of each SI word (two
bytes long) is the instruction byte, which goes into the
Instruction Register. The Instruction Register points the
second byte to its destination. In a typical application,
CS
is raised after one word (16 bits) to implement the
desired changes. Section 5.3, “Registers”, covers
applications using multiple 16-bit words. SO goes low
after CS
goes high; it has a push-pull output that does
not go into a high-Z state.
The MCP6S21/2/6/8 devices operate in SPI Modes 0,0
and 1,1. In 0,0 mode, the clock idles in the low state
(Figure 5-1) and, in 1,1 mode, the clock idles in the high
state (Figure 5-2). In both modes, SI data is loaded into
the PGA on the rising edge of SCK and SO data is
clocked out on the falling edge of SCK. In 0,0 mode, the
falling edge of CS
also acts as the first falling edge of
SCK (see Figure 5-1). There must be multiples of 16
clocks (SCK) while CS is low or commands will abort
(see Section 5.3, “Registers”).
FIGURE 5-1: Serial bus sequence for the PGA; SPI 0,0 mode (see Figure 1-5).
FIGURE 5-2: Serial bus sequence for the PGA; SPI 1,1 mode (see Figure 1-6).
12345678910 11 12 13 14 15 16
bit 7
CS
SCK
SI
Instruction Byte Data Byte
bit 0
bit 7 bit 0
SO
(first 16 bits out are always zeros)
12345678910111213141516
bit 7
CS
SCK
SI
Instruction Byte Data Byte
bit 0 bit 7 bit 0
SO
(first 16 bits out are always zeros)