Embedded clocks
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Jim Granville on 13 Aug 2006 15:31 rickman wrote: > Jim Granville wrote: > >>rickman wrote: >> >> >>>Jim Granville wrote: >>> >>> >>>>anything better than RC, has starting time issues, so usually runs >>>>all the time, and that has power penalties. >>> >>> >>>RC is not even an oscillator without other componets so it really is >>>not a solution. I can get an oscillator that runs on 1 mA of current, >>>costs under $0.50 and has plenty of accuracy to do any of the above >>>protocols. So async serial is ok. One in and one out. >> >>RC osc would use the CPLD - not sure what 'other components' you mean. >> >>If you are happy with 1mA and 50c, then that's fine. >> >>I see in my notes, Core ICC figures of ~20uA @ 15KHz for a CPLD RC osc, >>at a cost of a few cents. ( and appx 50uA at 1MHz ) > > > How do you get a CPLD to reliably oscillate with an RC? Choose one with Schmitt pin options (also important if you want to try i2c, where the slow edges will mess with non-schmitt CPLDs ). That's Atmel's ATF1502BE (32MC), or CoolrunnerII devices. You can make 1 pin, 2 pin, or 3 pin Oscillators - the more pins, the better the precision, and less it depends on the thresholds. The numbers I gave are for ATF1502BE. -jg
From:
Jim Granville on 13 Aug 2006 15:36 rickman wrote: <snip> > It just occured to me that I really don't need to know the format of > the data if I just treat the signals as arbitrary data streams. I can > sample them at high enough rates that the timing information is not > significantly distorted and send them across in a very, very simple > scheme. I can sample each one in a round-robin manner at say 1 MHz for > a total clock rate of 10 MHz (10 signals in one direction, 2 in the > other). This interface would need a clock, datain, dataout and frame > sync. If you use Sync related direction control ( see SPI memories ), you can share/common the datain/dataout : eg the first 12 clocks are one way, the last 4 are the other way, in a simple 16 clock frame, with spare room ) -jg
From: JustJohn on 13 Aug 2006 17:04 rickman wrote: > > How do you get a CPLD to reliably oscillate with an RC? > If you can afford 2 more CPLD pins, Peter described it for an FPGA, but should work for CPLD: http://www.xilinx.com/xlnx/xweb/xil_tx_display.jsp?sTechX_ID=pa_six_easy&iLanguageID=1&iCountryID=1 I'm curious: 1) how well does that circuit scale up in frequency, 2) what your frequency needs are? As described, using 470K resistors, the power for that 3 pin RC oscillator looks miniscule. Add a little more gating, you can even shut it off, and re-start time doesn't look much more than half a clock period.
From:
Jim Granville on 13 Aug 2006 17:50 JustJohn wrote: > rickman wrote: > >>How do you get a CPLD to reliably oscillate with an RC? >> > > > If you can afford 2 more CPLD pins, Peter described it for an FPGA, but > should work for CPLD: > > http://www.xilinx.com/xlnx/xweb/xil_tx_display.jsp?sTechX_ID=pa_six_easy&iLanguageID=1&iCountryID=1 > > I'm curious: > 1) how well does that circuit scale up in frequency, > 2) what your frequency needs are? > > As described, using 470K resistors, the power for that 3 pin RC > oscillator looks miniscule. RC Osc power is not just resistor power; a significant amount is the linear-region Icc from the pin buffers that are not square wave driven. > Add a little more gating, you can even shut it off, and re-start time > doesn't look much more than half a clock period. Restart time is instant, but you do need to watch the start(idle) values as the first half cycle time can differ from the others, as the circuit changes from rest state, to the ping-pong oscillation cycles. With 3 terminal Osc's, you should also design the Pinout/PCB, so the 'linear' node is adajcent to the positive acting pin(cap), and not the negative acting pin. eg a good pinout is : GND RC Cap+ Res- -jg
From: rickman on 13 Aug 2006 23:06
Jim Granville wrote: > rickman wrote: > > How do you get a CPLD to reliably oscillate with an RC? > > Choose one with Schmitt pin options (also important if you want to try > i2c, where the slow edges will mess with non-schmitt CPLDs ). > > That's Atmel's ATF1502BE (32MC), or CoolrunnerII devices. > > You can make 1 pin, 2 pin, or 3 pin Oscillators - the more pins, > the better the precision, and less it depends on the thresholds. > > The numbers I gave are for ATF1502BE. The ATF15xx and Coolrunner II are no gos because of the dual voltages required. Also the ATF parts only come in the relatively huge TQFP44 packages which are four times larger than the csBGA of the Coolrunner parts. I can make a schmitt trigger easily enough with a couple of resistors and an output pin. How about some details on the RC oscillator? I have not seen a 3 pin oscillator before. |