Mark K Sullivan
I have built a software Bell 201 (FSK) modem modulator using the 16C57 and a
demodulator with the 16C71 (for the A/D). The modulator would fit in a '54 but
I needed the '57 RAM for the rest of the application. My application is
half-duplex HDLC and I have to do the HDLC CRC as well as another software UART.
I think I could get full duplex in the '71 if I didn't have to do the software
I think you're going to have trouble with the 'C84 mostly because of the 10MHz
clock ceiling. I think you can modulate and maybe demodulate but not full
duplex. How were you going to digitize the incoming audio? In my modulator,
I have a software PWM for audio out. Hardware PWM or an external D/A would
greatly reduce the computing load. It should be easy to get a full duplex modem
in the 16C73, for instance.
It's nice to do development with an EEPROM. If you find you can't get the data
rate, you might consider doing your development with everything at half speed
and then moving to an EPROM part for the real system.
I do my (phase coherent) FSK modulation with a cosine lookup table. For FSK
demodulation, there are (at least) two choices. First, you can use a pair of
bandpass filters at the FSK tone frequencies followed by rectifiers, low-pass
filters, and a comparator. Another method is to delay the signal by 90 degrees
and multiply the original signal by the delayed version of itself, then low-pass
filter the product. In my case I use the latter technique and just feed the
sign of the product into the low-pass.
PSK would be easy to generate using the same technique but I don't know off the
top of my head how you would demodulate it. Does anyone else know? You might
look at TI and Motorola DSP application notes for a clue.
Does anyone have experience with clocking the 16C84 faster than 10MHz? I know
the EPROM parts will typically got 30 or even 40 MHz although I wouldn't try it
Good luck and keep us posted with your project. I'd like to know if PSK is
workable or if you come up with an alternative to the 16C7x A/D, which gets
- Mark Sullivan -
> I have built a software Bell 201 (FSK) modem modulator using the 16C57 and a
> demodulator with the 16C71 (for the A/D). The modulator would fit in a '54
> I needed the '57 RAM for the rest of the application. My application is
> half-duplex HDLC and I have to do the HDLC CRC as well as another software
> I think I could get full duplex in the '71 if I didn't have to do the
> UART concurrently.
> Does anyone have experience with clocking the 16C84 faster than 10MHz? I know
> the EPROM parts will typically got 30 or even 40 MHz although I wouldn't try
> in production.
I was looking at using some 16C62 or similar parts for an application
but due to no immediate stock I did the testing with C84 parts.
I wanted to run at 20 MHz but found that the C84 worked at 16 MHz
and sometimes (same IC even) at 18 MHz and never at 20 MHz all
this was done with an external ocillator can in HS mode.
I expect the Microchip people hoped to get the speed to 20 MHz but
the technology of the day could not do it reliably. Perhaps they
will come out with another EEPROM data memory device one of these
days with OTP code memory and 1 kByte of EEPROM data. Never had any
luck with getting prices on the dual chip uP plus serial EEPROM
devices. Though I notice they have some new ones with a 16C58
core and 1 kB memory.
Kalle Pihlajasaari data.co.zakalle
Interface Products Box 15775, Doornfontein, 2028, South Africa
+27 (11) 402-7750 Fax: +27 (11) 402-7751
Ing. Pablo Otero
At 10:07 AM 9/8/96 -0500, you wrote:
>I have built a software Bell 201 (FSK) modem modulator using the 16C57 and a
>demodulator with the 16C71 (for the A/D).
>- Mark Sullivan -
Would you like to share your code with the list it sound really great to have a
modem built in the pic and still have space for the application, instead of
a separate modem chip.
_/ N A F Electronic _/
_/ M e x i c o _/
_/ e-mail: mail.giga.com _/ nafpoc
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