MCP3424

Arduino library for 18 bit ADC I2C MCP3424 and compatibles.

Description

Experimental

Library not tested with hardware yet.

This library is to be used to configure and read the 18 bit MCP4324 4 channel ADC et al.

The MCP3424 is not a fast ADC, however with 18 bit it has at least a very high resolution. What the effects of the long sampling time means is to be investigated. The high resolution combined with an optional gain of 8x means one could measure voltage in steps of about 2 µV.

The library cannot check yet if a conversion is ready. Need hardware to check how this works in detail.

The library has three functions that return a reading in Volts, milliVolts or microVolts to match the need of the user.

The user has to configure the ADC device (bits, gain) and can call read() (et al) without parameters to keep usage straightforward.

Current implementation will probably change slightly in the future when related devices will be supported. (See future section).

Alt-230 = µ

Resolution

The effective resolution also depends on the gain set. In theory with a gain of 8 the LSB of the 18 bit resolution represents 1/8 of 15.625 µV == 1.95 µV. If this is feasible in practice is to be seen.

I2C Address

The MCP3421 and MCP3426 have a fixed address 0x68, one can order different addresses at the factory (how?).

The other devices have two address pins to set 8 addresses. The trick is to leave address pins floating. See datasheet table 5.3 for details.

I2C Speed

The MCP342x devices support 100 KHz, 400 KHz and 3.4 MHz. The latter is not (yet) supported by the library. The sketch MCP3424_performance.ino can be used to get some insight in the performance. It will check up to 800 kHz.

TODO verify with hardware.

I2C multiplexing

Sometimes you need to control more devices than possible with the default address range the device provides. This is possible with an I2C multiplexer e.g. TCA9548 which creates up to eight channels (think of it as I2C subnets) which can use the complete address range of the device.

Drawback of using a multiplexer is that it takes more administration in your code e.g. which device is on which channel. This will slow down the access, which must be taken into account when deciding which devices are on which channel. Also note that switching between channels will slow down other devices too if they are behind the multiplexer.

• https://github.com/RobTillaart/TCA9548

Related

• https://gammon.com.au/adc tutorial about ADC's (UNO specific)
• https://github.com/RobTillaart/ADS1x15 (12 & 16 bit ADC, I2C, slow)
• https://github.com/RobTillaart/PCF8591 (8 bit ADC + 1 bit DAC)
• https://github.com/RobTillaart/MCP_DAC SPI based ADC.
• https://github.com/RobTillaart/MCP3424 18 bit ADC, I2C

Interface

#include "MCP3424.h"

Constructor

• MCP3424(uint8_t address = 0x68, TwoWire *wire = &Wire)
• bool begin() initializes the device. POR?
• bool isConnected() checks if the device address can be seen on I2C bus.
• uint8_t getAddress() idem, convenience function.
• uint8_t getMaxChannels() idem, convenience function.

The MCP3421/2/3/6/7/8 constructors have the same parameters.

Read

• int32_t read() returns the raw reading.
• float readVolts() converts the raw reading to volts value (wrapper).
• float readMilliVolts() converts the raw reading to millivolts value (wrapper). This is useful for small ranges.
• float readMicroVolts() converts the raw reading to microvolts value (wrapper). This is useful for very small ranges (especially with a gain of 8 one has a resolution of about 2 microvolts.

Configuration

The user has to configure the ADC device (bits, gain) and can call read() (et al) without parameters to keep usage straightforward.

Correct settings will be written to the device immediately, but be aware of the fact that it will take some time before the conversion with new settings is done.

• bool setChannel(uint8_t channel = 0) not to be used for the MCP3421 as it has only one channel. Default is channel 0, parameter should be less than the value of getMaxChannels().
• uint8_t getChannel() returns chosen channel (default 0).
• bool setGain(uint8_t gain = 1) set gain to 1,2,4, or 8. Other values will return false and not change the setting.
• uint8_t getGain() returns the set gain (default 1).
• bool setResolution(uint8_t bits = 12) set the bit resolution 12,14,16 or 18. Other values will return false and not change the setting.
• uint8_t getResolution() returns the set resolution (default 12).
• void setContinuousMode() idem.
• void setSingleShotMode() idem.
• uint8_t getMode() returns 0 for singleShot and 1 for continuous.

The set function write their changes directly to the device. It might be better to have one function to set all parameters in one call. To be investigated.

The library caches the last configuration, it is not read back from the device. This might be added in the future.

Future

Must

• investigate reading of ready flag
• get hardware to test.
  • redo interface for MCP3424 if needed.
• investigate continuous vs single shot mode.
• improve documentation
  • Table of addresses.

Should

• test on different boards.
• optimize performance if possible
• check performance I2C with HW
• optimize setting all configuration in one function call.
  • setConfig(channel, resolution, gain, mode) ?
  • getter needed?
• implement PowerOnReset function for configuration

Could

• implement maxResolution (combine with maxChannels? in one "maxValue" byte)
  • check range in setResolution().
• extract gain and resolution from the config byte to reduce storage.
  • like getMode().
• extend examples
  • array of ADC's
  • mcp3424_plotter
• add error handling
  • add error variable
  • check return value writeConfig().
  • check read() process.

Wont

• cache last read value. (difficult for wrappers)

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