I'm 90% through with a software transmit only modulator.. application is half-duplex telemetry. I went with software method: A) To keep parts count/power usage down, B) MX614s are getting scarce. This is roughly equivalent to the single-chip APRS trackers, but more flexible as I can modify the source to suit _my_ needs, and besides, I'm used to AVRs, not pics.
Current code takes up about 1K of flash and ~20 bytes of ram/register (14 bytes static + stack) on an 8535. Unoptimized C code, 10 khz sample rate, DDS similar to the miniDDS design. I haven't decided, but I may release the source under the GPL or LGPL.
The other idea I am tossing around is a complete software modem; the megas have enough oomph for a couple of fixed-point FIR filters, and that's all you really need to demodulate the data (well, an opamp and RS-232 level shifter too; the lower 2-3 bits of the onchip ADCs are not the most accurate.) AND Atmel just released the Mega8535, DIP package, 5V, 16 mhz, hardware mult, $5-6 quantity _1_ from digikey... Sample at 8 khz.
It could easily buffer a complete packet either send or receive; if you are careful programming (ASM probably) you may be able to keep working data in registers and buffer two packets in ram.
Another option is the butterfly, once it is actually available. That's based on the mega169, has 512 KByte of extra data flash, LCD, rs232 shifter, joystick input, and piezo speaker. That's $20.. add case, connectors, and opamps, $30-40 for a nice standalone logging TNC kit. The only major trouble is it appears to be using the 4 mhz internal RC oscillator, which is not known for temperature stability or accuracy, but it does have a 32khz RTC to calibrate against as well as a temperature sensor onboard.
1200 baud is not that hard.
Any comments?
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Topic: 1200 baud APRS TNC KIT (Read 4905 times)