I need some clarification about what Atmel tries to explain in the datasheets..
Introduction (lengthy but necessary) :
Every AVR with BOD has a table with BOD threshold fuse settings voltages (aka V_bot). Some models have min/typical/max values given (like m16), while some (like usb162) give only typical values (whatever "typical" means).
Additionally Atmel performs one (tn2313, usb162), two(m1284p), three(m329A) etc. production BOD tests with each chip and they give information that even if V_bot is lower than that presented on "Speed vs. Vcc" chart, chip passed the test an is still guaranteed to be operational down to V_bot, whatever that voltage really is.
The problem is in most of the datasheets they do not give the information on how fast the chip was clocked in these tests and what Atmel really guarantees.
Example:ATMega32A rev.B, page 299:
ATMega32A speed grades chart is given as 8MHz at min. 2,7V and 16MHz with 4,5V (to 5,5V). Linear in between 2,7V and 4,5V.
With which F_CPU did they test the chip? If "typical voltage"=="test voltage" of 4V, I cannot clock m32A more than 13,7MHz with this BOD then.
I can buy a bunch of m329A (lowest V_bot, varies from 1.7V to 2V, 1.8V typical) and select those chips which are BOD reset at 1.7V (4MHz, including EEPROM/FLASH programming and ADC?). And among those there are some chips with BOD triggered with 2V (those still are guaranteed to be operational at 1.8V@4MHz, but without this BOD). If chips differ only in V_bot and chips do work at 1,7V@4MHz, why didn't Atmel specify it on the first page?
AT90USBxx2 and MegaxU2 series operating voltage range.
I am building a USB device with AT90USB162 chip. It is going to be high-power bus-powered (which means it must be USB operational down to 4,4V on upstream port plug when not configured and down to 4,75V when configured):
High-power functions must be capable of operating in their low-power (one unit load) mode with an input voltage as low as 4.40 V,(...) They must also be capable of operating at full power (up to five unit loads) with a VBUS voltage of 4.75 V
Another of my requirements is I need to run the core with a 16MHz clock while in high-power mode (when it is configured and device draws full current of 0,5A). So the 16MHz quartz is inevitable.
I can run cpu with lower clock (divide by 2) when it is not configured yet and to switch to 16MHz afterward. The problem is this device does not have a BOD setting, which guarantees a 16MHz operation (4,5V minimum is needed) as the highest setting triggers from 4.1V to 4.5V with "typical" at 4,3V. I cannot build a device which runs from 13MHz to 16MHz with "typical" F_CPU about 14,5MHz..
Modern models (U2 series) are tested with "typical" V_bot at 4,3V which falls somewhere below 15MHz.. Great, because USB is not operational then.
Can somebody explain how to use these USB chips within specification limits? Do I need a switched two level external BOD for that?