X-Git-Url: https://git.gag.com/?a=blobdiff_plain;f=AltOS%2Fdoc%2Fmicropeak.html;h=0c141c701908873f7ad27a9828d77e27e6d3c8b5;hb=23d7a60a20365d6c9b3e6f2a58f6a353de8dd593;hp=63348a4fafa7b67c3c2180b54d3be82a20830962;hpb=2d80b95d20d6010dde1d987beae3c92d4d9e1f56;p=web%2Faltusmetrum diff --git a/AltOS/doc/micropeak.html b/AltOS/doc/micropeak.html index 63348a4..0c141c7 100644 --- a/AltOS/doc/micropeak.html +++ b/AltOS/doc/micropeak.html @@ -1,386 +1,326 @@ -
Copyright © 2012 Bdale Garbee and Keith Packard
- This document is released under the terms of the - - Creative Commons ShareAlike 3.0 - - license. -
Revision History | |
---|---|
Revision 0.1 | 29 October 2012 |
- Initial release with preliminary hardware. - | |
Revision 1.0 | 18 November 2012 |
- Updates for version 1.0 release. - | |
Revision 1.1 | 12 December 2012 |
- Add comments about EEPROM storage format and programming jig. - | |
Revision 1.2 | 20 January 2013 |
- Add documentation for the MicroPeak USB adapter board. Note - the switch to a Kalman filter for peak altitude - determination. - | |
Revision 1.3.2 | 12 February 2014 |
- Add a "Download" button to the main window, which makes it - quicker to access the download function. Update the data - download documentation to reflect the new MicroPeak USB - adapter design. Monitor data during download to let you see - if the USB connection is working at all by showing the - characters received from the MicroPeak USB adapter. - |
- Thanks to John Lyngdal for suggesting that we build something like this. -
- Have fun using these products, and we hope to meet all of you - out on the rocket flight line somewhere. -
-Bdale Garbee, KB0G
-NAR #87103, TRA #12201
-
-Keith Packard, KD7SQG
-NAR #88757, TRA #12200
-
-
Table of Contents
List of Tables
- MicroPeak is designed to be easy to use. Requiring no external - components, flying takes just a few steps -
- Install the battery. Fit a CR1025 battery into the plastic - carrier. The positive (+) terminal should be towards the more - open side of the carrier. Slip the carrier into the battery - holder with the positive (+) terminal facing away from the - circuit board. -
![]() |
- Install MicroPeak in your rocket. This can be as simple as - preparing a soft cushion of wadding inside a vented model payload - bay. Wherever you mount it, make sure you protect the - barometric sensor from corrosive ejection gasses as those - will damage the sensor, and shield it from light as that can - cause incorrect sensor readings. -
- Turn MicroPeak on. Slide the switch so that the actuator - covers the '1' printed on the board. MicroPeak will report - the maximum height of the last flight in decimeters using a - sequence of flashes on the LED. A sequence of short flashes - indicates one digit. A single long flash indicates zero. The - height is reported in decimeters, so the last digit will be - tenths of a meter. For example, if MicroPeak reports 5 4 4 - 3, then the maximum height of the last flight was 544.3m, or - 1786 feet. -
- Finish preparing the rocket for flight. After the - previous flight data have been reported, MicroPeak waits for - one minute before starting to check for launch. This gives - you time to finish assembling the rocket. As those - activities might cause pressure changes inside the airframe, - MicroPeak might accidentally detect boost. If you need to do - anything to the airframe after the one minute window passes, - make sure to be careful not to disturb the altimeter. The - LED will remain dark during the one minute delay, but after - that, it will start blinking once every 3 seconds. -
- Fly the rocket. Once the rocket passes about 30m in height - (100 feet), the micro-controller will record the ground - pressure and track the pressure seen during the flight. In - this mode, the LED flickers rapidly. When the rocket lands, - and the pressure stabilizes, the micro-controller will record - the minimum pressure pressure experienced during the flight, - compute the height represented by the difference in air - pressure and blink that value out on the LED. After that, - MicroPeak powers down to conserve battery power. -
- Recover the data. Turn MicroPeak off and then back on. MicroPeak - will blink out the maximum height for the last flight. Turn - MicroPeak back off to conserve battery power. -
- All Altus Metrum products are sophisticated electronic devices. - When handled gently and properly installed in an air-frame, they - will deliver impressive results. However, as with all electronic - devices, there are some precautions you must take. -
- The CR1025 Lithium batteries have an - extraordinary power density. This is great because we can fly with - much less battery mass... but if they are punctured - or their contacts are allowed to short, they can and will release their - energy very rapidly! - Thus we recommend that you take some care when handling MicroPeak - to keep conductive material from coming in contact with the exposed metal elements. -
- The barometric sensor used in MicroPeak is sensitive to - sunlight. Please consider this when designing an - installation. Many model rockets with payload bays use clear - plastic for the payload bay. Replacing these with an opaque - cardboard tube, painting them, or wrapping them with a layer of - masking tape are all reasonable approaches to keep the sensor - out of direct sunlight. -
- The barometric sensor sampling ports must be able to "breathe", - both by not being covered by foam or tape or other materials that might - directly block the hole on the top of the sensor, and also by having a - suitable static vent to outside air. -
- As with all other rocketry electronics, Altus Metrum altimeters must - be protected from exposure to corrosive motor exhaust and ejection - charge gasses. -
Table of Contents
![]() |
- MicroPeak stores barometric pressure information for the first - 48 seconds of the flight in on-board non-volatile memory. The - contents of this memory can be downloaded to a computer using - the MicroPeak USB adapter. -
- The MicroPeak application runs on Linux, Mac OS X and - Windows. You can download the latest version from - http://altusmetrum.org/AltOS. -
- On Mac OS X and Windows, the FTDI USB device driver needs to - be installed. A compatible version of this driver is included - with the MicroPeak application, but you may want to download a - newer version from http://www.ftdichip.com/FTDrivers.htm. -
- Plug the MicroPeak USB adapter in to your computer. -
- Start the MicroPeak application. -
- Click on the Download button at the top of the window. -
![]() |
- Select from the listed devices. There will probably be - only one. -
![]() |
- The application will now wait until it receives valid data - from the MicroPeak USB adapter. -
![]() |
- The MicroPeak USB adapter has a small phototransistor - under the hole in the center of the box. - Locate this, turn on the MicroPeak and place the orange LED on the MicroPeak - directly inside the hole, resting the MicroPeak itself on - the box. You should see the blue LED on the MicroPeak USB - adapter blinking in time with the orange LED on the - MicroPeak board itself. -
![]() |
- After the maximum flight height is reported, MicroPeak will - pause for a few seconds, blink the LED four times rapidly - and then send the data in one long blur on the LED. The - MicroPeak application should receive the data. When it does, - it will present the data in a graph and offer to save the - data to a file. If not, you can power cycle the MicroPeak - board and try again. -
![]() |
- Once the data are saved, a graph will be displayed with - height, speed and acceleration values computed from the - recorded barometric pressure data. See the next section - for more details on that. -
- The MicroPeak application can present flight data in the form - of a graph, a collection of computed statistics or in tabular - form. -
- MicroPeak collects raw barometric pressure data which is - then used to compute the remaining data. Altitude is computed - through a standard atmospheric model. Absolute error in this - data will be affected by local atmospheric - conditions. Fortunately, these errors tend to mostly cancel - out, so the error in the height computation is much smaller - than the error in altitude would be. -
- Speed and acceleration are computed by first smoothing the - height data with a Gaussian window averaging filter. For speed - data, this average uses seven samples. For acceleration data, - eleven samples are used. These were chosen to provide - reasonably smooth speed and acceleration data, which would - otherwise be swamped with noise. -
- The File menu has operations to open existing flight logs, - Download new data from MicroPeak, Save a copy of the flight - log to a new file, Export the tabular data (as seen in the Raw - Data tab) to a file, change the application Preferences, Close - the current window or close all windows and Exit the - application. -
- Under the Graph tab, the height, speed and acceleration values - are displayed together. You can zoom in on the graph by - clicking and dragging to sweep out an area of - interest. Right-click on the plot to bring up a menu that will - let you save, copy or print the graph. -
![]() |
- The Statistics tab presents overall data from the flight. Note - that the Maximum height value is taken from the minumum - pressure captured in flight, and may be different from the - apparant apogee value as the on-board data are sampled twice - as fast as the recorded values, or because the true apogee - occurred after the on-board memory was full. Each value is - presented in several units as appropriate. -
![]() |
- A table consisting of the both the raw barometric pressure - data and values computed from that for each recorded time. -
![]() |
![]() |
- The MicroPeak application has a few user settings which are - configured through the Preferences dialog, which can be - accessed from the File menu. -
- The Log Directory is where flight data will be saved to - and loaded from by default. Of course, you can always - navigate to other directories in the file chooser windows, - this setting is just the starting point. -
- If you prefer to see your graph data in feet and - miles per hour instead of meters and meters per second, - you can select Imperial Units. -
- To see what data is actually arriving over the serial - port, start the MicroPeak application from a command - prompt and select the Serial Debug option. This can be - useful in debugging serial communication problems, but - most people need never choose this. -
- You can adjust the size of the text in the Statistics tab - by changing the Font size preference. There are three - settings, with luck one will both fit on your screen and - provide readable values. -
- The Look & feel menu shows a list of available - application appearance choices. By default, the MicroPeak - application tries to blend in with other applications, but - you may choose some other appearance if you like. -
-
- Note that MicroPeak shares a subset of the AltosUI - preferences, so if you use both of these applications, change - in one application will affect the other. -
Table of Contents
- MicroPeak uses the Measurement Specialties MS5607 sensor. This - has a range of 120kPa to 1kPa with an absolute accuracy of - 150Pa and a resolution of 2.4Pa. -
- The pressure range corresponds roughly to an altitude range of - -1500m (-4900 feet) to 31000m (102000 feet), while the - resolution is approximately 20cm (8 inches) near sea level and - 60cm (24in) at 10000m (33000 feet). -
- Ground pressure is computed from an average of 16 samples, - taken while the altimeter is at rest. The flight pressure used to - report maximum height is computed from a Kalman filter - designed to smooth out any minor noise in the sensor - values. The flight pressure recorded to non-volatile storage - is unfiltered, coming directly from the pressure sensor. -
- MicroPeak uses an Atmel ATtiny85 micro-controller. This tiny - CPU contains 8kB of flash for the application, 512B of RAM for - temporary data storage and 512B of EEPROM for non-volatile - storage of previous flight data. -
- The ATtiny85 has a low-power mode which turns off all of the - clocks and powers down most of the internal components. In - this mode, the chip consumes only .1μA of power. MicroPeak - uses this mode once the flight has ended to preserve battery - power. -
- The CR1025 battery used by MicroPeak holds 30mAh of power, - which is sufficient to run for over 40 hours. Because - MicroPeak powers down on landing, run time includes only time - sitting on the launch pad or during flight. -
- The large positive terminal (+) is usually marked, while the - smaller negative terminal is not. Make sure you install the - battery with the positive terminal facing away from the - circuit board where it will be in contact with the metal - battery holder. A small pad on the circuit board makes contact - with the negative battery terminal. -
- Shipping restrictions may prevent us from including a CR1025 - battery with MicroPeak. If so, many stores carry CR1025 - batteries as they are commonly used in small electronic - devices such as flash lights. -
- MicroPeak contains a fixed atmospheric model which is used to - convert barometric pressure into altitude. The model was - converted into a 469-element piece-wise linear approximation - which is then used to compute the altitude of the ground and - apogee. The difference between these represents the maximum - height of the flight. -
- The model assumes a particular set of atmospheric conditions, - which, while a reasonable average, cannot represent the changing - nature of the real atmosphere. Fortunately, for flights - reasonably close to the ground, the effect of this global - inaccuracy are largely canceled out when the computed ground - altitude is subtracted from the computed apogee altitude, so - the resulting height is more accurate than either the ground - or apogee altitudes. -
- Because the raw pressure data is recorded to non-volatile - storage, you can use that, along with a more sophisticated - atmospheric model, to compute your own altitude values. -
- MicroPeak is designed to be rugged enough for typical rocketry - applications. It contains two moving parts, the battery holder - and the power switch, which were selected for their - ruggedness. -
- The MicroPeak battery holder is designed to withstand impact - up to 150g without breaking contact (or, worse yet, causing - the battery to fall out). That means it should stand up to - almost any launch you care to try, and should withstand fairly - rough landings. -
- The power switch is designed to withstand up to 50g forces in - any direction. Because it is a sliding switch, orienting the - switch perpendicular to the direction of rocket travel will - serve to further protect the switch from launch forces. -
- The ATtiny85 has 512 bytes of non-volatile storage, separate - from the code storage memory. The MicroPeak firmware uses this - to store information about the last completed - flight. Barometric measurements from the ground before launch - and at apogee are stored, and used at power-on to compute the - height of the last flight. -
- In addition to the data used to present the height of the last - flight, MicroPeak also stores barometric information sampled - at regular intervals during the flight. This is the - information captured with the MicroPeak USB adapter. It can - also be read from MicroPeak through any AVR programming - tool. -
Table 4.1. MicroPeak EEPROM Data Storage
Address | Size (bytes) | Description |
---|---|---|
0x000 | 4 | Average ground pressure (Pa) |
0x004 | 4 | Minimum flight pressure (Pa) |
0x008 | 2 | Number of in-flight samples |
0x00a … 0x1fe | 2 | Instantaneous flight pressure (Pa) low 16 bits |
- All EEPROM data are stored least-significant byte first. The - instantaneous flight pressure data are stored without the - upper 16 bits of data. The upper bits can be reconstructed - from the previous sample, assuming that pressure doesn't - change by more more than 32kPa in a single sample - interval. Note that this pressure data is not - filtered in any way, while both the recorded ground and apogee - pressure values are, so you shouldn't expect the minimum - instantaneous pressure value to match the recorded minimum - pressure value exactly. -
- MicroPeak samples pressure every 96ms, but stores only every - other sample in the EEPROM. This provides for 251 pressure - samples at 192ms intervals, or 48.192s of storage. The clock - used for these samples is a factory calibrated RC circuit - built into the ATtiny85 and is accurate only to within ±10% at - 25°C. So, you can count on the pressure data being accurate, - but speed or acceleration data computed from this will be - limited by the accuracy of this clock. -
- MicroPeak exposes a standard 6-pin AVR programming interface, - but not using the usual 2x3 array of pins on 0.1" - centers. Instead, there is a single row of tiny 0.60mm × - 0.85mm pads on 1.20mm centers exposed near the edge of the - circuit board. We couldn't find any connector that was - small enough to include on the circuit board. -
- In lieu of an actual connector, the easiest way to connect to - the bare pads is through a set of Pogo pins. These - spring-loaded contacts are designed to connect in precisely - this way. We've designed a programming jig, the MicroPeak - Pogo Pin board which provides a standard AVR interface on one - end and a recessed slot for MicroPeak to align the board with - the Pogo Pins. -
- The MicroPeak Pogo Pin board is not a complete AVR programmer, - it is an interface board that provides a 3.3V regulated power - supply to run the MicroPeak via USB and a standard 6-pin AVR - programming interface with the usual 2x3 grid of pins on 0.1" - centers. This can be connected to any AVR programming - dongle. -
- The AVR programming interface cannot run faster than ¼ of the - AVR CPU clock frequency. Because MicroPeak runs at 250kHz to - save power, you must configure your AVR programming system to - clock the AVR programming interface at no faster than - 62.5kHz, or a clock period of 32µS. -
Copyright © 2014 Keith Packard
+ This document is released under the terms of the + + Creative Commons ShareAlike 3.0 + + license. +
Thanks to John Lyngdal for suggesting that we build something +like this.
Have fun using these products, and we hope to meet all of you +out on the rocket flight line somewhere.
Bdale Garbee, KB0G
+NAR #87103, TRA #12201
Keith Packard, KD7SQG
+NAR #88757, TRA #12200
Table of Contents
List of Figures
List of Tables
MicroPeak is designed to be easy to use. Requiring no external +components, flying takes just a few steps
MicroPeak stores barometric pressure information for the first +48 seconds of the flight in on-board non-volatile memory. The +contents of this memory can be downloaded to a computer using +the MicroPeak USB adapter.
The MicroPeak application runs on Linux, Mac OS X and +Windows. You can download the latest version from +http://altusmetrum.org/MicroPeak
On Mac OS X and Windows, the FTDI USB device driver +needs to be installed. A compatible version of this +driver is included with the MicroPeak application, but +you may want to download a newer version from +http://www.ftdichip.com/FTDrivers.htm
The MicroPeak application can present flight data in +the form of a graph, a collection of computed +statistics or in tabular form.
MicroPeak collects raw barometric pressure data which +is then used to compute the remaining data. Altitude +is computed through a standard atmospheric +model. Absolute error in this data will be affected by +local atmospheric conditions. Fortunately, these +errors tend to mostly cancel out, so the error in the +height computation is much smaller than the error in +altitude would be.
Speed and acceleration are computed by first smoothing +the height data with a Gaussian window averaging +filter. For speed data, this average uses seven +samples. For acceleration data, eleven samples are +used. These were chosen to provide reasonably smooth +speed and acceleration data, which would otherwise be +swamped with noise.
The File menu has operations to open existing flight +logs, Download new data from MicroPeak, Save a copy of +the flight log to a new file, Export the tabular data +(as seen in the Raw Data tab) to a file, change the +application Preferences, Close the current window or +close all windows and Exit the application.
Under the Graph tab, the height, speed and acceleration values +are displayed together. You can zoom in on the graph by +clicking and dragging to sweep out an area of +interest. Right-click on the plot to bring up a menu that will +let you save, copy or print the graph.
The Statistics tab presents overall data from +the flight. Note that the Maximum height value +is taken from the minumum pressure captured in +flight, and may be different from the apparant +apogee value as the on-board data are sampled +twice as fast as the recorded values, or +because the true apogee occurred after the +on-board memory was full. Each value is +presented in several units as appropriate.
A table consisting of the both the raw barometric pressure +data and values computed from that for each recorded time.
The MicroPeak application has a few user settings which are +configured through the Preferences dialog, which can be +accessed from the File menu.
Note that MicroPeak shares a subset of the +AltosUI preferences, so if you use both of +these applications, change in one application +will affect the other.
All Altus Metrum products are sophisticated electronic +devices. When handled gently and properly installed in an +air-frame, they will deliver impressive results. However, as +with all electronic devices, there are some precautions you +must take.
The CR1025 Lithium batteries have an extraordinary power +density. This is great because we can fly with much less +battery mass⦠but if they are punctured or their contacts +are allowed to short, they can and will release their energy +very rapidly! Thus we recommend that you take some care when +handling MicroPeak to keep conductive material from coming in +contact with the exposed metal elements.
The barometric sensor used in MicroPeak is sensitive to +sunlight. Please consider this when designing an +installation. Many model rockets with payload bays use clear +plastic for the payload bay. Replacing these with an opaque +cardboard tube, painting them, or wrapping them with a layer +of masking tape are all reasonable approaches to keep the +sensor out of direct sunlight.
The barometric sensor sampling ports must be able to +"breathe", both by not being covered by foam or tape or other +materials that might directly block the hole on the top of the +sensor, and also by having a suitable static vent to outside +air.
As with all other rocketry electronics, Altus Metrum +altimeters must be protected from exposure to corrosive motor +exhaust and ejection charge gasses.
MicroPeak uses the Measurement Specialties MS5607 +sensor. This has a range of 120kPa to 1kPa with an +absolute accuracy of 150Pa and a resolution of 2.4Pa.
The pressure range corresponds roughly to an altitude +range of -1500m (-4900 feet) to 31000m (102000 feet), +while the resolution is approximately 20cm (8 inches) +near sea level and 60cm (24in) at 10000m (33000 feet).
Ground pressure is computed from an average of 16 +samples, taken while the altimeter is at rest. The +flight pressure used to report maximum height is +computed from a Kalman filter designed to smooth out +any minor noise in the sensor values. The flight +pressure recorded to non-volatile storage is +unfiltered, coming directly from the pressure sensor.
MicroPeak uses an Atmel ATtiny85 +micro-controller. This tiny CPU contains 8kB of flash +for the application, 512B of RAM for temporary data +storage and 512B of EEPROM for non-volatile storage of +previous flight data.
The ATtiny85 has a low-power mode which turns off all +of the clocks and powers down most of the internal +components. In this mode, the chip consumes only .1μA +of power. MicroPeak uses this mode once the flight has +ended to preserve battery power.
The CR1025 battery used by MicroPeak holds 30mAh of +power, which is sufficient to run for over 40 +hours. Because MicroPeak powers down on landing, run +time includes only time sitting on the launch pad or +during flight.
The large positive terminal (+) is usually marked, +while the smaller negative terminal is not. Make sure +you install the battery with the positive terminal +facing away from the circuit board where it will be in +contact with the metal battery holder. A small pad on +the circuit board makes contact with the negative +battery terminal.
Shipping restrictions may prevent us from including a +CR1025 battery with MicroPeak. If so, many stores +carry CR1025 batteries as they are commonly used in +small electronic devices such as flash lights.
MicroPeak contains a fixed atmospheric model which is +used to convert barometric pressure into altitude. The +model was converted into a 469-element piece-wise +linear approximation which is then used to compute the +altitude of the ground and apogee. The difference +between these represents the maximum height of the +flight.
The model assumes a particular set of atmospheric +conditions, which, while a reasonable average, cannot +represent the changing nature of the real +atmosphere. Fortunately, for flights reasonably close +to the ground, the effect of this global inaccuracy +are largely canceled out when the computed ground +altitude is subtracted from the computed apogee +altitude, so the resulting height is more accurate +than either the ground or apogee altitudes.
Because the raw pressure data is recorded to +non-volatile storage, you can use that, along with a +more sophisticated atmospheric model, to compute your +own altitude values.
MicroPeak is designed to be rugged enough for typical +rocketry applications. It contains two moving parts, +the battery holder and the power switch, which were +selected for their ruggedness.
The MicroPeak battery holder is designed to withstand +impact up to 150g without breaking contact (or, worse +yet, causing the battery to fall out). That means it +should stand up to almost any launch you care to try, +and should withstand fairly rough landings.
The power switch is designed to withstand up to 50g +forces in any direction. Because it is a sliding +switch, orienting the switch perpendicular to the +direction of rocket travel will serve to further +protect the switch from launch forces.
MicroPeak exposes a standard 6-pin AVR programming +interface, but not using the usual 2x3 array of pins +on 0.1" centers. Instead, there is a single row of +tiny 0.60mm à 0.85mm pads on 1.20mm centers exposed +near the edge of the circuit board. We couldnât find +any connector that was small enough to include on the +circuit board.
In lieu of an actual connector, the easiest way to +connect to the bare pads is through a set of Pogo +pins. These spring-loaded contacts are designed to +connect in precisely this way. Weâve designed a +programming jig, the MicroPeak Pogo Pin board which +provides a standard AVR interface on one end and a +recessed slot for MicroPeak to align the board with +the Pogo Pins.
The MicroPeak Pogo Pin board is not a complete AVR +programmer, it is an interface board that provides a +3.3V regulated power supply to run the MicroPeak via +USB and a standard 6-pin AVR programming interface +with the usual 2x3 grid of pins on 0.1" centers. This +can be connected to any AVR programming dongle.
The AVR programming interface cannot run faster than ¼ +of the AVR CPU clock frequency. Because MicroPeak runs +at 250kHz to save power, you must configure your AVR +programming system to clock the AVR programming +interface at no faster than 62.5kHz, or a clock period +of 32µS.
The ATtiny85 has 512 bytes of non-volatile storage, separate +from the code storage memory. The MicroPeak firmware uses this +to store information about the last completed +flight. Barometric measurements from the ground before launch +and at apogee are stored, and used at power-on to compute the +height of the last flight.
In addition to the data used to present the height of the last +flight, MicroPeak also stores barometric information sampled +at regular intervals during the flight. This is the +information captured with the MicroPeak USB adapter. It can +also be read from MicroPeak through any AVR programming tool.
Table C.1. MicroPeak EEPROM Data Storage
Address | Size (bytes) | Description |
0x000 | 4 | Average ground pressure (Pa) |
0x004 | 4 | Minimum flight pressure (Pa) |
0x008 | 2 | Number of in-flight samples |
0x00a ⦠0x1fe | 2 | Instantaneous flight pressure (Pa) low 16 bits |
All EEPROM data are stored least-significant byte first. The +instantaneous flight pressure data are stored without the +upper 16 bits of data. The upper bits can be reconstructed +from the previous sample, assuming that pressure doesnât +change by more more than 32kPa in a single sample +interval. Note that this pressure data is not filtered in +any way, while both the recorded ground and apogee pressure +values are, so you shouldnât expect the minimum instantaneous +pressure value to match the recorded minimum pressure value +exactly.
MicroPeak samples pressure every 96ms, but stores only every +other sample in the EEPROM. This provides for 251 pressure +samples at 192ms intervals, or 48.192s of storage. The clock +used for these samples is a factory calibrated RC circuit +built into the ATtiny85 and is accurate only to within ±10% at +25°C. So, you can count on the pressure data being accurate, +but speed or acceleration data computed from this will be +limited by the accuracy of this clock.