rtcfifo Module

The rtcfifo module implements a first-in,first-out storage intended for sensor readings. As the name suggests, it is backed by the RTC user memory and as such survives deep sleep cycles. Conceptually it can be thought of as a cyclic array of { timestamp, name, value } tuples. Internally it uses a space-optimized storage format to allow the greatest number of samples to be kept. This comes with several trade-offs, and as such is not a one-solution-fits-all. Notably: - Timestamps are stored with second-precision. - Sample frequency must be at least once every 8.5 minutes. This is a side-effect of delta-compression being used for the time stamps. - Values are limited to 16 bits of precision, but have a separate field for storing an E-n multiplier. This allows for high fidelity even when working with very small values. The effective range is thus 1E-7 to 65535. - Sensor names are limited to a maximum of 4 characters.

Important:

This module uses two sets of RTC memory slots, 10-20 for its control block, and a variable number of slots for samples and sensor names. By default these span 32-127, but this is configurable. Slots are claimed when rtcfifo.prepare() is called.

This is a companion module to the rtcmem and rtctime modules.

rtcfifo.dsleep_until_sample()

When the rtcfifo module is compiled in together with the rtctime module, this convenience function is available. It allows for some measure of separation of concerns, enabling writing of modularized Lua code where a sensor reading abstraction may not need to be aware of the sample frequency (which is largely a policy decision, rather than an intrinsic of the sensor). Use of this function is effectively equivalent to rtctime.dsleep_aligned(interval_us, minsleep_us) where interval_us is what was given to rtcfifo.prepare().

Syntax

rtcfifo.dsleep_until_sample(minsleep_us)

Parameter

minsleep_us minimum sleep time, in microseconds

Example

-- deep sleep until it's time to take the next sample
rtcfifo.dsleep_until_sample(0)

See also

rtctime.dsleep_aligned()

rtcfifo.peek()

Reads a sample from the rtcfifo. An offset into the rtcfifo may be specified, but by default it reads the first sample (offset 0).

Syntax:

rtcfifo.peek([offset])

Parameters

offset Peek at sample at position offset in the fifo. This is a relative offset, from the current head. Zero-based. Default value is 0.

Returns

The values returned match the input arguments used to rtcfifo.put().

  • timestamp timestamp in seconds
  • value the value
  • neg_e scaling factor
  • name sensor name

If no sample is available (at the specified offset), nothing is returned.

Example

local timestamp, value, neg_e, name = rtcfifo.peek()

rtcfifo.pop()

Reads the first sample from the rtcfifo, and removes it from there.

Syntax:

rtcfifo.pop()

Parameters

none

Returns

The values returned match the input arguments used to rtcfifo.put().

  • timestamp timestamp in seconds
  • value the value
  • neg_e scaling factor
  • name sensor name

Example

while rtcfifo.count() > 0 do
  local timestamp, value, neg_e, name = rtcfifo.pop()
  -- do something with the sample, e.g. upload to somewhere
end

rtcfifo.prepare()

Initializes the rtcfifo module for use.

Calling rtcfifo.prepare() unconditionally re-initializes the storage - any samples stored are discarded.

Syntax

rtcfifo.prepare([table])

Parameters

This function takes an optional configuration table as an argument. The following items may be configured:

  • interval_us If wanting to make use of the rtcfifo.sleep_until_sample() function, this field sets the sample interval (in microseconds) to use. It is effectively the first argument of rtctime.dsleep_aligned().
  • sensor_count Specifies the number of different sensors to allocate name space for. This directly corresponds to a number of slots reserved for names in the variable block. The default value is 5, minimum is 1, and maximum is 16.
  • storage_begin Specifies the first RTC user memory slot to use for the variable block. Default is 32. Only takes effect if storage_end is also specified.
  • storage_end Specified the end of the RTC user memory slots. This slot number will not be touched. Default is 128. Only takes effect if storage_begin is also specified.

Returns

nil

Example

-- Initialize with default values
rtcfifo.prepare()
-- Use RTC slots 19 and up for variable storage
rtcfifo.prepare({storage_begin=21, storage_end=128})

See also

rtcfifo.ready()

See also

rtcfifo.prepare()

rtcfifo.put()

Puts a sample into the rtcfifo.

If the rtcfifo has not been prepared, this function does nothing.

Syntax

rtcfifo.put(timestamp, value, neg_e, name)

Parameters

  • timestamp Timestamp in seconds. The timestamp would typically come from rtctime.get().
  • value The value to store.
  • neg_e The effective value stored is valueEneg_e.
  • name Name of the sensor. Only the first four (ASCII) characters of name are used.

Note that if the timestamp delta is too large compared to the previous sample stored, the rtcfifo evicts all earlier samples to store this one. Likewise, if name would mean there are more than the sensor_count (as specified to rtcfifo.prepare()) names in use, the rtcfifo evicts all earlier samples.

Returns

nil

Example

-- Obtain a sample value from somewhere
local sample = ... 
-- Store sample with no scaling, under the name "foo"
rtcfifo.put(rtctime.get(), sample, 0, "foo")

rtcfifo.ready()

Returns non-zero if the rtcfifo has been prepared and is ready for use, zero if not.

Syntax:

rtcfifo.ready()

Parameters

none

Returns

Non-zero if the rtcfifo has been prepared and is ready for use, zero if not.

Example

-- Prepare the rtcfifo if not already done
if not rtcfifo.ready() then
  rtcfifo.prepare()
end