Datasheet

© 2009 Microchip Technology Inc. DS21950E-page 21
MCP3550/1/3
5.0 SERIAL INTERFACE
5.1 Overview
Serial communication between the microcontroller and
the MCP3550/1/3 devices is achieved using CS, SCK
and SDO/RDY. There are two modes of operation:
Single Conversion and Continuous Conversion. CS
controls the conversion start. There are 24 bits in the
data word: 22 bits of conversion data and two overflow
bits. The conversion process takes place via the
internal oscillator and the status of this conversion
must be detected. The typical method of
communication is shown in Figure 5-1. The status of
the internal conversion is the SDO/RDY
pin and is
available with CS
low. A High state on SDO/RDY
means the device is busy converting, while a Low
state means the conversion is finished and data is
ready for transfer using SCK. SDO/RDY
remains in a
high-impedance state when CS is held high. CS must
be low when clocking out the data using SCK and
SDO/RDY
.
Bit 22 is Overflow High (OVH) when V
IN
> V
REF
– 1 LSB,
OVH toggles to logic ‘1’, detecting an overflow high in
the analog input voltage.
Bit 23 is Overflow Low (OVL) when V
IN
< -V
REF
, OVL
toggles to logic ‘1’, detecting an overflow low in the
analog input voltage. The state OVH = OVL = ‘1’ is not
defined and should be considered as an interrupt for
the SPI interface meaning erroneous communication.
Bit 21 to bit 0 represents the output code in 22-bit
binary two's complement. Bit 21 is the sign bit and is
logic ‘0’ when the differential analog input is positive
and logic ‘1’ when the differential analog input is
negative. From Bit 20 to bit 0, the output code is given
MSb first (MSb is bit 20 and LSB is Bit 0). When the
analog input value is comprised between -V
REF
and
V
REF
– 1 LSB, the two overflow bits are set to logic ‘0’.
The relationship between input voltage and output
code is shown in Figure 5-1.
The delta-sigma modulator saturation point for the
differential analog input is located at around ±112% of
V
REF
(at room temperature), meaning that the
modulator will still give accurate output codes with an
overrange of 12% below or above the reference
voltage. Unlike the usual 22-bit device, the 22-bit out-
put code will not lock at 0x1FFFFF for positive sign
inputs or 0x200000 for negative sign inputs in order to
take advantage of the overrange capabilities of the
device. This can be practical for closed-loop
operations, for instance. In case of an overflow, the
output code becomes a 23-bit two's complement output
code, where the sign bit will be the OVL bit. If an
overflow high or low is detected, OVL (bit 23) becomes
the sign bit (instead of bit 21), the MSb is then bit 21
and the converter can be used as a 23-bit two's
complement code converter, with output code from bits
B21 to B0, and OVL as the sign bit. Figure 5-1
summarizes the output coding data format with or
without overflow high and low.
FIGURE 5-1: Typical Serial Device Communication and Example Digital Output Codes for Specific
Analog Input Voltages.
CS
SCK
D
O
O
21 20 19 18 17 16 15 14 13 12 11 10 9
8
765
4
3
21
0
READY
H
L
R
HI-Z
SDO/RDY