Difference between revisions of "ADC max1027"

Latest revision as of 17:03, 18 February 2011

Introduction

The MAX1027 is an 8 channels 10 bits analog-to-digital converter (ADC) with an internal reference and an internal temperature sensor.
Only 7 channels can be used (AIN0-AIN6) when AIN7 is configured as "start of conversion". As the MAX1027 is a quite complicated component to drive (because of its versatility), we recommand to read its datasheet before going further.

Driver

Depending on your needs (ie sampling slow or fast signals) there are 2 ways to use the max1027 Linux driver:

For slow signals (temperature or battery level monitoring):

The driver can be strictly controlled from its /sys interface: /sys/bus/spi/devices/spiN.0/ (N depending on the number of SPI devices declared for your target).
Moreover, if needed (lm-sensors & Co), you can add it a Linux Hardware Monitoring API (hwmon) compatible/standardized interface: /sys/class/hwmon/hwmonN/device/ (N depending on the ID given by hwmon). This interface mimics the /sys/bus/spi/devices/ one.

For "fast" signals (<10kHz):

The driver has a /dev interface which can be configured through /sys/bus/spi/devices/spiX.0/.

Installation

Max1027 driver is now by default installed on standard rootfs. so these instructions are kept here as reference.

Usage

• On the target, the driver can be started like this:

 # modprobe max1027
 max1027 v0.5 successfully probed !

Note: In the following chapters, ADC_SPI will refer to spi1.0 in case of an APF9328 and to spi0.0 in case of the APF27Dev. So for convenience you can do: # export ADC_SPI="spi1.0" (on APF9328) or # export ADC_SPI="spi0.0" (on APF27DevFull)

"Slow" & configuration interface

Several interfaces in /sys/bus/spi/devices/spi... are created in order to:

• read the 7 analog inputs values: inx_input (in mV)
• read the temperature: temp1_input (in m°C)
• modify the max1027 registers: conversion, setup & averaging

 # ls /sys/bus/spi/devices/$ADC_SPI/
 averaging    driver       in2_input    in5_input    modalias     temp1_input
 bus          in0_input    in3_input    in6_input    setup        uevent
 conversion   in1_input    in4_input    in7_input    subsystem

An input can be read like that:

 # cat /sys/bus/spi/devices/$ADC_SPI/in0_input

The temperature can be read like that:

 # cat /sys/bus/spi/devices/$ADC_SPI/temp1_input

Note: These values are only updated each time you launch a conversion, ie access the MAX1027 conversion register, after having configured setup register accordingly. So this interface is only suitable to measure slow evolving greats. Example: # let set=0x62; echo $set > /sys/bus/spi/devices/$ADC_SPI/setup (configure MAX1027; needed only once) # let conv=0x87; echo $conv > /sys/bus/spi/devices/$ADC_SPI/conversion (launch 1 conversion of temp & AN0) # cat /sys/bus/spi/devices/$ADC_SPI/temp1_input 32750 (m°C) # cat /sys/bus/spi/devices/$ADC_SPI/in0_input 1135 (mV)

"Fast" interface (frequencies > 10Hz)

In that case you can use the /dev interface (using blocking read to wait end of conversion). To create /dev nodes you can use the loadmax.sh script located in armadeus/target/linux/modules/max1027 (driver's MAJOR number is dynamically allocated, so /dev needs to be dynamically created).

 # loadmax.sh

• if not already installed:

 $ cp target/linux/modules/max1027/loadmax.sh /tftpboot/

 # tftp -g -r loadmax.sh 192.168.0.X    (192.168.0.X = your Host PC's IP address)
 # sh loadmax.sh

• first, the conversion mode has to be chosen (channel 0->6 and temp):

 # let conv=0xb1; echo $conv > /sys/bus/spi/devices/$ADC_SPI/conversion

• and eventually the averaging and setup registers as well (refer to the max1027 documentation for more details):

 # let set=0x48; echo $set > /sys/bus/spi/devices/$ADC_SPI/setup
 # let avg=0x20; echo $avg > /sys/bus/spi/devices/$ADC_SPI/averaging

• then you can for example repeatedly read value of analog input 0 (variable sampling rate = depends on system load):

 # cat /dev/max1027/AIN0 | hexdump

• read analog input 0 once:

 # time dd if=/dev/max1027/AIN0 bs=2 count=1 > tmp.bin
 # hexdump tmp.bin

This mode is activated by default in the Max1027 driver. If you switched to "slow" mode you can activate "fast" mode again with:

 # let set=0x48; echo $set > /sys/bus/spi/devices/$ADC_SPI/setup

Chip registers configuration

Independent of the mode, /sys interfaces allow direct access to the corresponding register in the MAX1027:

 # let conv=0xb1; echo $conv > /sys/bus/spi/devices/$ADC_SPI/conversion

You can also get current (hexadecimal) values of conversion, averaging and setup registers with (for example):

 # cat /sys/bus/spi/devices/$ADC_SPI/setup
 0x62

For more details on how to use these registers, take a look at the datasheet.

Shell examples

• Get channel 0 and temperature values:

 # let conv=0x87; echo $conv > /sys/bus/spi/devices/$ADC_SPI/conversion
 # cat /sys/class/hwmon/hwmon0/device/temp1_input
 # cat /sys/class/hwmon/hwmon0/device/in0_input

• Get channels [0-6] and temperature values:

 # let conv=0xb1; echo $conv > /sys/bus/spi/devices/$ADC_SPI/conversion

• Get 4 average values (of 32 conversions) for channel 0:

 # let avg=0x3c; echo $avg > /sys/bus/spi/devices/$ADC_SPI/averaging
 # let conv=0x84; echo $conv > /sys/bus/spi/devices/$ADC_SPI/conversion

MAX1027 on APF27Dev board

The MAX1027 is optionnaly mounted on the APF27Dev board. It is ready to use and the ADC inputs can be accessed through J6

figure 1 - MAX1027 localisation on APF27Dev

MAX1027 on APF9328 board

It is mounted (in option) on the APF9328 module. To know if you have one on your APF938, take a look at the connectors side of the module:

figure 2 - MAX1027 localisation on APF9328

If you don't have one and are a good electrician, you can order a sample at Maxim's website and solder it directly on your board (footprint is available).

Hardware interface

Warning: The 8 ADC inputs voltage range is [0 - 3,3V]. Don't try to put higher voltage !!

By default internal reference voltage is used (2,5V), so if you apply analog values > 2,5V, they will be truncated.

figure 3 - MAX1027 connections

On APF9328

The i.MXL SPI_1 interface is used to communicate with the device.

Warning: In the original design we thought using AIN7 as analog input (instead of #CNVST) and thought too we won't need #EOC; but due to some chip bugs, it is now required. So if you want to use the MAX1027 then you will have to connect #EOC pin (End Of Conversion) and #CNVST pin (conversion start) to two i.MXL GPIOs.

By default the driver is using:

• PA13 (CSI_HSYNC) for #EOC
• and PA14 (CSI_PIXCLK) for #CNVST

So for example on the APF9328DevFull board you will have to connect:

• pin 11 of ADC connector (right of the Ethernet one) to pin 12 of CSI connector (below RS232 one)
• and pin 10 of the ADC connector to pin 9 of the CSI connector

On APF27Dev

Nothing special to do: MAX1027 is already set in "fast mode" and using #EOC and #CNVST pins.

Performances

Links

• MAX1027 datasheet on Maxim website
• hwmon documentation