[time-nuts] CPLDs for clock dividers

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Feb 3 21:20:47 UTC 2010

The internal state machine oscillator behaviour for Xilinx 9500 series 
CPLDs is given here:


Bruce Griffiths wrote:
> One issue to bear in mind is subharmonic phase modulation due to the 
> currents associated with lower frequency outputs flowing in the ground 
> and supply leads.
> Another not well known issue is that modern CPLDs have an internal 
> usually undocumented state machine and associated oscillator to 
> intialise the logic from the internal EEPROM.
> This is done to reduce power consumption as EEPROM cells have 
> significant dc current.
> The EEPROM is disabled after the initialisation, however the state 
> machine internal oscillator may not be.
> To find out if the state machine oscillator is turned off after the 
> intialisation process you have to ask the manufacturer.
> The state machine oscillator is an unwanted source of phase modulation 
> whenever it is oscillating.
> Bruce
> paul swed wrote:
>> They are indeed cheap and it would be handy to have low noise divider
>> chains.
>> 6 decades worth. 74ls90s get really boring to wire.
>> Unfortunately I am unfamiliar with using the device. But I do seem them
>> used.
>> On Wed, Feb 3, 2010 at 1:59 PM, Matt Ettus<boyscout at gmail.com>  wrote:
>>> Does anyone have any experience using CPLDs for very low phase noise
>>> dividers?  You can get an XC9536XL from Xilinx for around $1, and I
>>> thought it would make a good divide by 2 through 10 device.
>>> Matt

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