AVR Frequency Meter

This is design for a frequency meter based on AVR microcontrollers. Maximum input frequency is specified to be 30 MHz in the multi-chip configuration, and in single-chip configuration, there are both 5 MHz and 10 Mhz versions operating with 10 and 20 MHz crystals, respectively. All versions have 9 1/2 digit resolution. I have used multi-chip version at 40 Mhz, and depending upon the actual chips you use and your layout, it can work at much higher input frequencies. The 10 MHz single-chip version using the ATtiny2313 is ideal for use with a X10 or X 100 prescaler.
AVR Frequency Meter - schematic

As for microcontroller choice, the requirements for this firmware are that the controller be an AVR with a 16 bit counter, has a ram stack and UART or USART, and the ability to operate at 10 MHz. Some modification may be required to the code to accommodate a USART instead of a UART, a 16 bit stack pointer instead of the 8 bit pointer on the AT90S2313, and the specific controls of the 16 bit counter registers. I used an AT90S2313 because they are available to me and suitably small. For the 10 MHz input single-chip configuration, you need to use an AVR that can clock at 20 MHz. I used an ATtiny2313-20. When programming the ATtiny2313, remember to select a fuse setting for the internal clock oscillator. The AT90S2313 does not have clock fuse settings. The first phase was to make a high resolution frequency meter/counter just using the AT90S2313 ot ATtiny2313, then to add the external prescaler. Without the prescaler, the maximum input frequency for the frequency meter is 5 MHz or 10 MHz, depending upon the chip you use and the firmware version you choose. With the multi-chip version that includes a prescaler, the maximum input frequency, according the the component specifications, is about 30 Mhz - this will vary with individual external prescaler chips and your circuit layout. To summarize, there are three versions of the code available at the top of this page: The 5 MHz single-chip version, which is basically an...

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