Maxim Integrated MAX32664A User manual

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Measuring Heart Rate and SpO2
Using the MAX32664A – A Quick Start Guide
UG7087; Rev 0; 8/19
Abstract
The MAX32664A is a variant of the MAX32664 sensor-hub family, which is specifically targeted
for the finger-based measurement of heart rate and SpO2. Combined with the MAX30101 optical
sensor and a 3-axis accelerometer, it provides a sensor’s raw data, as well as calculated heart-
rate and SpO2data to a host device through its I2C slave interface. This document provides
step-by-step instructions that enable a user to communicate with the MAX32664A, and to
calibrate, configure, and receive measurement and monitoring data.

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Table of Contents
Introduction ................................................................................................................................ 3
1 Architecture............................................................................................................................. 4
1.1 Communicating with the MAX32664A............................................................................... 5
1.2 Accelerometer .................................................................................................................. 6
2 Calibration of the SpO2Algorithm ........................................................................................... 7
2.1 Calibration of SpO2Coefficients for the Final Product ...................................................... 7
2.2 Algorithm Settings and Configurations.............................................................................. 8
3 Measuring Heart Rate and SpO2on Finger ...........................................................................11
3.1 Raw Data Collection Mode ..............................................................................................11
3.2 Algorithm Mode: Heart Rate and SpO2...........................................................................13
Revision History ........................................................................................................................15
List of Figures
Figure 1. Architecture diagram for health-sensing applications. ................................................. 4
List of Tables
Table 1. Read Status Byte Value ............................................................................................... 5
Table 2. Host-Side Accelerometer—Sending Data to the MAX32664A ...................................... 6
Table 3. Configurations and Settings—HR/SpO2........................................................................ 8
Table 4. Host Commands—SpO2Calibration............................................................................. 9
Table 5. Format of Received Samples—SpO2Calibration Mode...............................................10
Table 6. Host Commands—Raw Data Mode.............................................................................11
Table 7. Format of Received Samples—Raw Data Mode .........................................................12
Table 8. Host Commands—HR/SpO2Algorithm........................................................................13
Table 9. Format of Received Samples—HR/SpO2Algorithm ....................................................14

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Introduction
The MAX32664A is a variant of the MAX32664 sensor-hub family that enables users to capture
raw data as well as calculated heart-rate and SpO2data through finger contact. The MAX32664A
is preprogrammed with the firmware, drivers, and algorithm that are required to interface with the
MAX30101 sensor device through an I2C master port. The I2C slave interface is dedicated to
establishing communication with a host microcontroller.
In order to properly capture and calculate the data, it is recommended that accelerometer data be
provided to the MAX32664A. The MAX32664A firmware includes the required drivers for the
Kionix®KX122 accelerometer, which is wired together with the MAX30101 to the same I2C port.
Alternatively, a host-side accelerometer can be used. In this case, the sampled accelerometer
data must be periodically reported to the MAX32664A by the host microcontroller.
This document provides the instructions necessary to create a solution with the MAX32664A
based on the MAXREFDES220# reference design.
Kionix is a registered trademark of Kionix, Inc.

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1 Architecture
A typical health-sensing design includes a host microcontroller that communicates with the
MAX32664A through the I2C bus. Two GPIO pins are needed to control the reset and the startup
in Application or Bootloader mode through the RSTN and multifunction input/output (MFIO) pins.
An MFIO pin is also used in Application mode to interrupt the host for I2C communication. The
MAX32664A interfaces with the MAX30101 optical sensor through a second I2C bus.
To enter Bootloader mode:
•Set the RSTN pin to low for 10ms.
•While RSTN is low, set the MFIO pin to low. (The MFIO pin should be set to low at least
1ms before the RSTN pin is set to high.)
•After the 10ms has elapsed, set the RSTN pin to high.
•After an additional 50ms has elapsed, the MAX32664 is in Bootloader mode.
To enter Application mode:
•Set the RSTN pin to low for 10ms.
•While RSTN is low, set the MFIO pin to high.
•After the 10ms has elapsed, set the RSTN pin to high. (The MFIO pin should be set to
high at least 1ms before the RSTN pin is set to high.)
•After an additional 50ms has elapsed, the MAX32664 is in Application mode and the
application performs its initialization of the application software.
•After approximately 1 second from when the RSTN pin was set to high, the application
completes the initialization and the device is ready to accept I2C commands.
Figure 1 shows the top-level architecture.
Figure 1. Architecture diagram for health-sensing applications.

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1.1 Communicating with the MAX32664A
A host should use the I2C bus to communicate with the MAX32664A (slave) using a series of
commands. A generic write command includes the following fields:
Slave_WriteAddress(1 byte)|Command_Family(1 byte)|Command_Index(1
byte)|Value(multiple bytes)
A generic response includes the following fields:
Slave_ReadAddress(1 byte)|Status(1 byte)|Value(multiple bytes)
Slave_WriteAddress and Slave_ReadAddress are set to 0xAA and 0xAB, respectively.
The read status byte is an indicator of success (0x00) or failure, as shown Table 1.
Table 1. Read Status Byte Value
STATUS
BYTE VALUE
DESCRIPTION
0x00
The write transaction was successful.
0x01
Illegal Family Byte and/or Command Byte was used.
0x02
This function is not implemented.
0x03
Incorrect number of bytes sent for the requested Family Byte.
0x04
Illegal configuration value was attempted to be set.
0x05
Incorrect mode specified. (In bootloader: Device is busy. Try again.)
0x80
General error while receiving/flashing a page during the bootloader
sequence.
0x81
Checksum error while decrypting/checking page data.
0x82
Authorization error.
0x83
Application not valid.
0xFE
Device is busy. Try again.
0xFF
Unknown error.
This document provides examples of commands for establishing communication with the
MAX32664A. For a complete list of commands and instructions for the I2C interface, see the
MAX32664 User Guide.

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1.2 Accelerometer
For best results, it is recommended that accelerometer data be provided to the MAX32664A.
SpO2 calculation requires a resting condition, and the algorithm uses accelerometer data to detect
excessive motion. In such a condition, computation is paused, and the user is informed with a
motion flag.
A sensor hub accelerometer can be integrated through the I2C port of the MAX32664A. In this
case, the required driver for KX122 is already included. The user only needs to follow the
reference schematics to connect the accelerometer and enable it before starting the algorithm,
as described later in this document.
Alternatively, a host-side accelerometer can be used. In order to use the host-side accelerometer:
1. The host should start the accelerometer just before enabling the algorithm to maximize
the initial synchronization between the PPG and accelerometer samples. However,
accelerometer samples collected prior to receiving the confirmation of the algorithm
enable I2C command should be discarded.
2. The host is required to use a 3-axis accelerometer at a 100Hz sampling rate. If a higher
sampling rate is chosen, samples should be decimated to be synchronized with a 10ms
PPG sampling time.
3. The host must queue five accelerometer samples and feed them at the same time to the
MAX32664A using the commands shown in Table 2. The period of feeding samples
should be 200ms. Because the sensor and the host accelerometer use different clock
sources, exact synchronization between them is not possible.
Table 2. Host-Side Accelerometer—Sending Data to the MAX32664A
HOST COMMAND
(HEX) DESCRIPTION
MAX32664
RESPONSE
(HEX)
DESCRIPTION
AA 44 04 01 01
Enable the host accelerometer.
AB 00
Success
AA 13 00 04 Read the sensor sample size for
the accelerometer (optional). AB 00 06
Success; 6 is the
number of bytes per
samples in FIFO
The following should be executed periodically at 200ms:
AA 14 00 [Sample
1 values] …
[Sample N values]
Write data to the input FIFO of the
sensor hub.
Each sample has three 2-byte
integer values for X, Y, and Z in
milli-g.
N = 20
AB 00 Success
AA 00 00 Read the sensor hub status. AB 00 00
Success; sensor hub
not busy

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2 Calibration of the SpO2Algorithm
2.1 Calibration of SpO2Coefficients for the Final Product
Due to variations in the physical design and optical shield of the final product, a calibration
procedure is required to be performed once in a controlled environment. This procedure is
important to ensure the quality of the SpO2 calculation. This step is typically performed in a
standard lab with a reference SpO2device to determine three calibration coefficients: a, b, and c.
The details of the calibration procedure are described in the Guidelines for SpO2 Measurement
Using the Maxim MAX32664 Sensor Hub application note.
Once three calibration coefficients are obtained, they need to be loaded to the MAX32664A every
time prior to starting the algorithm. But first, they are required to be converted to a 32-bit integer
format using the following:
•Aint32 = round (105x a)
•Bint32 = round (105x b)
•Cint32 = round (105x c)
For example, the default measured calibration coefficients are:
•a = 1.5958422
•b = -34.659664
•c = 112.68987
They are sent to the MAX32664A in integer format after conversion:
•Aint32 = round (105x a) = 0x00026F60
•Bint32 = round (105x b) = 0xFFCB1D12
•Cint32 = round (105x c) = 0x00ABF37B
The calibration coefficients may be stored in the host flash separately and loaded to the
MAX32664A after every reset.
Table 4 shows the sequence of commands for the calibration process. Table 5 shows the format
of received samples. Typically, R values are needed for the calibration process, as described in
the Guidelines for SpO2 Measurement Using the Maxim MAX32664 Sensor Hub application
note.

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2.2 Algorithm Settings and Configurations
Table 3shows the settings that are available for the heart rate (HR)/SpO2algorithm. To update
the algorithm settings, be sure to send the appropriate commands BEFORE enabling the
algorithm, as shown in Table 8.
Table 3. Configurations and Settings—HR/SpO2
FAMILY
BYTE
ALGORITHM
INDEX
CONFIGURATION
INDEX
DESCRIPTION
DEFAULT
VALUE
0x50 for
write
0x51 for
read
0x02 0x0B
SpO
2
calibration coefficients*
100,000 (12 bytes comprised of
three 32-bit signed values)
A = 1.5958422
(0x00026f60)
B = -34.659664
(0xffcb1d12)
C = 112.68987
(0x00abf37b)

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Table 4 shows the list of commands to start the SpO2calibration.
Table 4. Host Commands—SpO2Calibration
#
HOST COMMAND
(HEX)
COMMAND DESCRIPTION
RESPONSE
(HEX)
START ALGORITHM
Host initializes the MAX32664A in calibration mode and starts the algorithm using following
commands:
1.1
AA 10 00 03
Set the output mode to sensor + algorithm
data (0x03) (streamed data will include PPG,
accelerometer, and algorithm data).
AB 00
1.2
AA 10 01 0F
Set the sensor hub interrupt threshold.
AB 00
1.3
AA 52 00 01
Enable the AGC.
AB 00
1.4
AA 44 04* 01 00 (if sensor
hub accelerometer is used)
AA 44 04* 01 01 (if Host
accelerometer is used)
Enable the accelerometer with the sensor
hub or host-side accelerometer.*
(Do not use this command if there is no
accelerometer.)
AB 00
1.5
AA 44 03* 01
Enable the AFE (e.g., the MAX30101).
AB 00
1.6
AA 52 02 02 (SpO
2
calibration report)
Enable the HR/SpO
2
algorithm. The format of
the samples is shown in Table 5.
AB 00
1.7
Wait for 100ms before sending the next command. Any command to change sensor
registers should appear AFTER enabling the algorithm or they will be overwritten.
READING SAMPLES
Host reads the samples upon receiving the MFIO interrupt from the MAX32664A. For SpO
2
calibration, continue as needed to capture the R value. See Table 5.
2.1
AA 00 00
Read the sensor hub status byte:
Bit 0: Sensor comm error
Bits 1 and 2: Reserved
Bit 3: FIFO filled to threshold (DataRdyInt)
Bit 4: Output FIFO overflow (FifoOutOvrInt)
Bit 5: Input FIFO overflow (FifoInOverInt)
Bit 6: Sensor hub busy (DevBusy)
Bit 7: Reserved
If DataRdyInt is set, proceed to the next step.
AB 00 08
2.2
AA 12 00
Get the number of samples (nn) in the FIFO.
AB 00 nn
2.3
AA 12 01
Read the data stored in the FIFO; nn
samples (30 bytes each) will be read. The
format of the samples is shown in Table 5.
AB 00
data_for_
nn_samples
STOP
Host ends the procedure:
3.1
AA 44 03* 00
Disable the AFE (e.g., the MAX30101).*
AB 00
3.2
AA 44 04* 00
Disable the accelerometer.* (Do not use this
command if there is no accelerometer.)
AB 00
3.3
AA 52 02 00
Disable the algorithm.
AB 00
*Provided indexes are example for sensors such as the MAX30101 AFE or KX122 accelerometer

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Table 5. Format of Received Samples—SpO2Calibration Mode
*If the output mode includes the sensor.
**If the output mode includes the algorithm.
DATA SOURCE BYTE INDEX DATA ITEM
# OF BYTES
(MSB FIRST)
DESCRIPTION
MAX30101
(12 Bytes)*
0
LED1
3
IR counter
3
LED2
3
Red counter
6
LED3
3
N/A
9
LED4
3
N/A
Accelerometer
(6 Bytes)*
12 accelX 2
Two's complement. LSB =
0.001g
14 accelY 2
Two's complement. LSB =
0.001g
16 accelZ 2
Two's complement. LSB =
0.001g
HR/SpO2
Algorithm
(12 Bytes)**
18
Heart rate
2
10x heart-rate value
20
Heart rate
confidence
1
Calculated confidence level
in %
21
SpO2
2
10x SpO2value
23 Algorithm
state 1
Algorithm current state:
0: No object is detected
1: Something is on sensor
2: Another object is detected
3: Finger is detected
24
R
2
10x calculated R value
26 Algorithm
status 1
Algorithm current status:
0: Success
1: Not ready
-1: Something is on sensor
-2: Device excessive motion
-3: No object
-4: Pressing too hard
-5: Object instead of finger
-6: Finger excessive motion
27
Reserved
3
Reserved