The internal feedback resistor
LoopyByteloose
Posts: 12,537
There was discussion on the use of resonators and crystals recently.
In fact, there has been quite a debate on how to get matched pairs and software alternatives.
Also, there was mention that it is best to have a feedback resistor in your completed·even though one is often omitted in benchwork.· I even emailed Gunther and asked if the added resistor would affect using the SX-Key or SX-Blitz [noparse][[/noparse]He said, 'Not that he knows of."]·as I was trying to decide if I should include it in board designs.
Finally, I came across an obscure reference to a 'fuse' setting {see Page 231 of Gunther's text} for an internal feedback resistor and a statement that the default setting was 'ON'.·
·Is this in all the SX products or was it an early feature that no longer is provided?
·Is there any document that indicates its actual value or value range?· [noparse][[/noparse]This is important because the value it likely to not be useful for the whole range of frequencies.]
I am wondering if those who are trying to gain 'tight frequency control' might benefit from turning off the fuse and inserting an adjustible potentometer to calibrate with a scope.
·[noparse][[/noparse]Please note that the following will not work and is pretty much nonsense.· - G. Herzog]
AND.. one additional 'by the way'.· I have been able to get 14pin DIP TTL oscillators, but the SX-48/52 protoboard provides only for the 8pin DIP TTL oscillators.· After looking at the schematic and the configuration, I see the 'back two pins are both ground and grounded to the case.
So, if you must, you can solder a bit of wire [noparse][[/noparse]paper clips or bus wire] to adapt a 14pin DIP to the 8pin DIP socket.
Obviously, the package will overhang the board, but you can get higher frequencies with the oscillators.
·
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G. Herzog in Taiwan
Post Edited (Kramer) : 10/9/2005 3:54:34 AM GMT
In fact, there has been quite a debate on how to get matched pairs and software alternatives.
Also, there was mention that it is best to have a feedback resistor in your completed·even though one is often omitted in benchwork.· I even emailed Gunther and asked if the added resistor would affect using the SX-Key or SX-Blitz [noparse][[/noparse]He said, 'Not that he knows of."]·as I was trying to decide if I should include it in board designs.
Finally, I came across an obscure reference to a 'fuse' setting {see Page 231 of Gunther's text} for an internal feedback resistor and a statement that the default setting was 'ON'.·
·Is this in all the SX products or was it an early feature that no longer is provided?·Is there any document that indicates its actual value or value range?· [noparse][[/noparse]This is important because the value it likely to not be useful for the whole range of frequencies.]I am wondering if those who are trying to gain 'tight frequency control' might benefit from turning off the fuse and inserting an adjustible potentometer to calibrate with a scope.
·[noparse][[/noparse]Please note that the following will not work and is pretty much nonsense.· - G. Herzog]
AND.. one additional 'by the way'.· I have been able to get 14pin DIP TTL oscillators, but the SX-48/52 protoboard provides only for the 8pin DIP TTL oscillators.· After looking at the schematic and the configuration, I see the 'back two pins are both ground and grounded to the case.
So, if you must, you can solder a bit of wire [noparse][[/noparse]paper clips or bus wire] to adapt a 14pin DIP to the 8pin DIP socket.
Obviously, the package will overhang the board, but you can get higher frequencies with the oscillators.
·
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
G. Herzog in Taiwan
Post Edited (Kramer) : 10/9/2005 3:54:34 AM GMT
Comments
in the meantime, I have gathered some more information about the SX-Key, and I can assure you that the SX-Key will not have problems with a feedback resistor between OSC1 and OSC2.
According to the SX datasheet, there is a bit in the FUSE register, the /IFBD bit which (when this and the /IRC bitis also set) enables an internal feedback resistor of about 1 MOhm. Similar to the weak pull-up resistors that can be activated for the port input pins, I would not rely on it.
The /IFBD bit is defined for all SX devices, and as there are no plans to change the SX silicon, this feature will be available in the future IMO. When checking the datasheets, I found that the SX 28 (and smaller) datasheet specifies the internal feedback resistor value being 1 MOhm where the SX48/52 datsheet specifies its value being 10 MOhm. I assume that this is a typo as 10 MOhm would be way too high, so I think the SX48/52 internal feedback resistor is also 1 MOhm. As mentioned before, I would not rely on this resistor anyway, therefore its absolute value does not matter at all.
Forget about inserting a trim potentiometer in the clock circuit. We deal here with frequencies up to 75 MHz, so each additional millimeter conducting the clock signal is good for generating EMI. Besides this, I doubt that adjusting the feedback resistor is a good method to fine-tune the clock signal. You also can't connect a scope to the clock signal for calibrating the frequency. Even an oscilloscope probe with attenuator would add enough parasitic capacitance to the clock line causing a frequency change, so you can't use it for calibrating this way.
Concerning you additional "by the way": The 8-pin "DIP socket" on the SX48/52 Proto Board is not intended for coscillator components. Instead, it allows to connect a parallel resistor, and/or capacitors (see the Proto Board schematic for the details).
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Greetings from Germany,
G
it is a good idea to indirectly measure the clock frequency. For example, you might write a small program that does nothing else but periodically toggle an output pin at the highest possible rate:
Loop
setb rb.0 ; 1
clrb rb.0 ; 1
jmp Loop ; 3
As you can see, it takes 5 clock cycles to execute the loop. IOW, the frequency of the signal at rb.0 is 1/5th of the clock frequency.
In my opinion, for most projects, a high-precision clock is not that important as one might assume unless you age going to implement really time-critical application, or want to synchronize two or more SXes at high speed.
Yes, the 4-pin socket on the SX48/52 Proto Board marked CLOCK can be used to plug in one of the commonly available clock generators. These devices require connections to Vdd, and Vss and generate the specified clock signal that is fed into the SXes OSC1 pin. When you turn the board with this four-pin socket located to the left, the contact top left is Vss, the contacts top right and bottom right are Vdd, and the contact bottom left is the oscillator output going the the SXes OSC1 pin.
The other "8-pin socket" can be used to plug in a crystal, capacitors between OSC1 and Vss and/or OSC2 and Vss, or to plug in an external feedback resistor.
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Greetings from Germany,
G