![]() ![]() This behaviour got me thinking about potential situations that may occur when TT7B is afloat. Apparently, in a situation where the MCU starts up on a voltage that is ramping up too slowly or is noisy, as it could be in solar powered trackers, the IC might lock and require an external reset. In a discussion on #highaltitude (IRC channel), Alan Adamson (HIRFW-6) has mentioned an issue with some of the Cortex-M0/M0+ microcontrollers. ![]() Completing the programming interface is VTref a line for sensing the target voltage, ground GND, and an access to the MCU's RESET pin which is internally pulled up. According to the datasheet, a 1kΩ pull-up resistor on the SWCLK line is critical for reliable operation. Programming of the MCU is carried out by Serial Wire Debug (SWD) interface which uses SWCLK and SWDIO lines located on pins PA30 and PA31, respectively. According to the datasheet it typically moves around 60-95♚/MHz in active mode and down to 1.2♚ in standby mode. The consumption of the MCU itself depends on a number of factors and settings. The TCXO will be powered continually with expected ~1.5♚. ![]() The other crystal input pins XIN and XOUT are used as general I/O pins. The datasheet also recommends leaving PA02 and PA03 as 'static' as possible, so they won't be utilized in the design. The clock signal is fed to XIN32 pin while XOUT32 is left unconnected. I've chosen the LVCMOS version that is specced as minimum high at 90% of VDD, maximum low at 10% of VDD with 48-52% duty cycle which is in compliance with the XOSC32K Digital Clock Characteristic in the MCU's datasheet. There are a few versions of the SiT1552 TCXO differing in their output. This temperature stable signal then serves either directly or as the basis for the MCU's internal clock generators which provide higher frequency programmable clocks to individual domains. The schematics and user guide of SAML21 Xplained Pro evaluation board are a useful source of information as well.Ī post by Bristol SEDS on their UBSEDS13 tracker informed me about the existence of very low power 32.768kHz TCXOs that can be used to provide a clock signal to a microcontroller. Its datasheet provides a basic check list on what needs to be done to make a custom implementation work. The 128kB flash and 32kB SRAM version, E17, seems like the best compromise between its cost and memory size for my objective. There are a few pin compatible variants: E15, E16, E17 and E18 which differ in SRAM and flash memory sizes. Due to its low power performance and my familiarity with the Atmel/Microchip environment, the 32 pin Cortex M0+ SAML21E series of MCUs was chosen to be in the middle of the tracker. It is also the component that will run continuously, hence its consumption is a factor to be taken into account. ![]() The microcontroller is the main middleman that needs to be equipped to be able to interface with all the other parts. With the basic concept in mind, I started choosing individual parts and outlining a schematic. These pdf files then contain detailed specification of the component, requirements on basic wiring, examples of application, recommended PCB mounting, etc. If not, a quick google search will most likely fetch it among the top results. These often tens or hundreds of pages long documents are usually linked to from the catalogues. Most of the information, though, can be found in a component's datasheet. The data might provide insight into the balloon's behaviour during its ascent, the day-night cycle, and potentially its descent as well. The backlogging will fill in the data for when the tracker is out of reach should the balloon survive to the next receiver hotspot. The coverage around the world is sufficiently predictable to further save power by limiting the transmissions to areas with active receivers only. The APRS transmissions are short enough not to waste too much power.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |