To: rsgb_lf_group@blacksheep.org
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From: Tobias DG3LV <DG3LV@gmx.net>
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Date: Fri, 12 Feb 2016 17:08:04 +0100
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Subject: Re: LF: PIC-based GPSDO with A/D converter / supported serial
 baudrates ?
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Hi Wolf !

Please have a look at :
http://www.ftdichip.com/Products/ICs/FT4232H.htm

it says : "RS232/RS422/RS485 UART Transfer Data Rate up to 12Mbaud. 
(RS232 Data Rate limited by external level shifter)."

The datasheet
http://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT4232H.pdf 
claims :
"Data Transfer rate: The FT4232H supports a data transfer rate up to 12 
Mbit/s when configured as an RS232/RS422/RS485 UART interface. Please 
note the FT4232H does not support the baud rates of 7 Mbaud 9 Mbaud, 10 
Mbaud and 11 Mbaud."

and :

"The Baud Rate Generators provides a x16 or a x10 clock input
to the UART’s from a 120MHz reference clock and consists of a 14 bit 
pre-scaler and 4 register bits which provide fine tuning of the baud 
rate (used to divide by a number plus a fraction). This determines the
Baud Rate of the UART which is programmable from 183 baud to 12 million 
baud."

If you can omit the (RS232-) level shifter and just use the 5 Volt 
TTL-Level in/output directly, then higher baudrates than 115200 should 
be possible at not too long cable-distances. (if possible, use 
RS422/RS485 differential signalling) At the QRL we did tests with this 
chip at up to 4 MBit some time ago with some success using the VCP 
driver. (There may be other chips from FTDI having a similar 
performance, we just selected this one.)

The FT4232H chip is, too, available as a (not-too-expensive, €28.71) 
mini-module at their webshop. (The more prominent chips from FTDI may be 
a target to fakes at the "usual market platforms")

The Windows(/Linux)-program on the PC has to be able to configure these 
baudrates. Standard Terminal-programs will support up to 115200 Baud. 
But self-created programs could go well beyond this (using the WIN32 -API).

73 de dg3lv Tobias

Am 12.02.2016 um 15:35 schrieb Wolfgang Büscher:
> Greetings all,
>
> The PIC-based GPSDO I am currently building (using a PIC16F1783, but
> maybe in the end it will turn out to be a dsPIC30F3013) is happily
> synchronizing the surplus 10 MHz VCOCXO for over a week now, without a
> single hiccup.
> So far, so good... I'm quite happy with the very low phase noise from
> the control loop (which uses two integrator to bring back the phase
> error to zero, should there ever be a 'hiccup' which has not happened yet).
> The next plan is to use the PIC's internal 12-bit ADC as a replacement
> for the soundcard, and possibly the PIC's internal PSMC as a simple
> downconverter ( programmable switch mode controller, can supply 40 MHz /
> N or 20 MHz / (N + M/16) with the fractional frequency adjustment ).
> To keep it simple, without an extra DDS, frequencies which the GPSDO can
> produce 'directly' (using the PSMC) are for example:
>
> 476190.47619048 Hz
> 475498.33887043 Hz
> 474806.20155039 Hz
> 474114.06423034 Hz
> 473421.92691030 Hz
> 472729.78959025 Hz
> 472037.65227021 Hz
>
>   (for the curious reader, the above frequenies are taken from the 10
> MHz OCXO, multiplied by the PIC's internal PLL to 40 MHz, then divided
> down by the PSMC as described in the PIC16F1782/3 datasheet,
> DS40001579E, on page 220. One could even use the PSMC as a crude BPSK
> modulator, but that's off topic for the moment).
>
> Since I don't want to develop an own USB driver for the digitized input,
> the interface shall use the UART (RS232 or RS422, this is up to the
> re-builder). The question is which serial baudrates (bits per second)
> are supported by the common "RS232" / USB adapters. They all support
> 115200 bit/sec but that's a bit low (if most of the digital
> downconversion shall be performed by the PC, due to the stepwidth of the
> PSMC output), so higher serial baudrates for the transmission of analog
> samples (from PIC to PC) are desirable. The question is WHICH higher
> baudrates are available, and is there a simple way on a PC (Linux or
> Windows, shouldn't matter).
> Must these all be multiples of 115200 ? So far I could only test it with
> the already mentioned 'Prolific' adapter, but I can see no way how to
> enumerate the baudrates which the adapter *really* supports.
> Trial-and-error experiments with various bitrates only produced garbage
> data. The rise and fall times at the RS232-TXD output are about 800 ns
> which is surprisingly steep so I guess the level converter itself cannot
> be the problem. Any ideas / recommendations ?
> Buy a 'good' FTDI adapter (instead of the Prolific), and if so, which
> one exactly ?
> If you have one of those, can you switch it to 1 MBit/second, hook it up
> to a scope, send a few characters from a terminal program and check the
> bit timing + rise & fall times ?
>
> 73,  Wolf DL4YHF .
>
>
>
>
>
>