Marc wrote:
> 
> > > Does anyone know how I might go about making a PIC into a frequency shifted keying modem?  I plan
> > > to use it to make some form of pager but I don't really know how to start..
> >
> > FSK generation won't be a problem, but the detection will be
> > a pain in the butt on a PIC.
> >
> > There is a application note on Scenix's web site for a complete
> > 1200 baud FSK modem. You can get the info from :
> > ftp://ftp.scenix.com/apps/fsk.src
> 
> Well I downloaded the file and it contains TX code only.
> 
> But I am sure it is no problem to implement FSK receive on a PIC, too.  After all it's just 1200
> bps. HAMCOM type modems contain a 1-bit A->D and 1 bit math does not require MUL or DIV
> instructions. Although HAMCOM type receivers are not perfect regarding their S/N requirements
> they are used a lot in amateur radio and people can live with it.

I managed to track down the news release. It is a reference 
design, so you might have to ask them for it.


Santa Clara, Calif. - Sept. 7, 1998 - Scenix Semiconductor, Inc.
has released a reference design that implements a fully functional 
1200-baud FSK/2400-baud DPSK modem in software using the Scenix SX 
Series 8-bit microcontroller (MCU). The modem is fully compliant 
with Bell 202 and CCITT V.22 bis standards. The reference design, 
which also includes DTMF generation and detection and Caller ID 
functions, allows engineers to reduce parts count and total system 
cost in embedded applications requiring data connectivity, including 
point-of-sale terminals, automatic teller machines, answering 
machines and remote data gathering and metering systems. The Scenix 
SX Series 50 MHz microcontroller is the only 8-bit MCU capable of 
fully executing modem functions in software. Until now, software- 
implemented modems required a relatively expensive 16- or 32-bit 
MCU or digital signal processor (DSP). Other 8-bit MCU solutions 
require an external modem chip, increasing both the parts count 
and total system cost. The modem reference design replaces external 
components with Scenix-designed "Virtual Peripherals." The reference 
design incorporates software modules that perform 1200-baud FSK, 
2400-baud DPSK and DTMF generation and detection, and Caller ID 
functions. Additional Virtual Peripherals, including a 19.2-kbit 
UART interface, a 16-bit timer, two pulse-width modulation (PWM) 
outputs and an analog-to-digital converter (ADC), are also part 
of the modem reference design. Also, because the SX MCU contains 
2K x 12 bits of flash/EEPROM, it can be reprogrammed in the field 
to incorporate new or enhanced feature sets. 

> 
> If you want to see it proven with code (PIC/SX/AVR), hire me :-)
For the above specs, SX sure, PIC/AVR no way.

> Regarding pagers: if they are POCSAG you don't need to demodulate the FSK yourself, as the data
> directly modulates the UHF carrier. Any FM receiver does the demodulation, so you only
> need a comparator (or preferably a treshold device with hysteresis) and a DPLL to recover
> the bit clock.

-- 
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