Interfacing DAC0832 with 80188

Discussion in 'Embedded Systems and Microcontrollers' started by oddabe83, Mar 31, 2007.

  1. oddabe83

    Thread Starter New Member

    Mar 31, 2007
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    Hi,
    currently I am doing a project that connects a dac0832 to a 80188. The bulk of my circuit uses 5V that comes out from a 7805. When I read the datasheet for the dac0832 (or dac0832 generally), the timing figures show that I'll need at least 9V to get the time widths I want. Especially, the write and XFER times are typically 375ns.

    The problem is the 80188 is running at 8mhz, which made each clock 125ns. Does this mean that it'll need 3 clock cycles to write to the DAC?
    Did anyone use DAC0832 with 5V before? Is it true that higher voltages implies better signal quality?

    TIA!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Are you sure the DAC0832 changes transmission speed with voltage? The National data sheet nowhere mentions 9 volts. Look closer at the timing charts.

    Are you sure the 80188 will connect directly to the DAC0832? Where do the control signals come from?

    Higher voltages have no effect on signal quality - that's a function of circuit design and layout. Possibly you mean resolution, which is a function of the number of bits in the converter.
     
  3. oddabe83

    Thread Starter New Member

    Mar 31, 2007
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    Thanks for replying!

    For this project, what I am actually trying to do is to connect the DAC to LM386, such that the output from LM386 produces a signal that drives a speaker.

    I refer to page 5 of the datasheet under AC characteristics, that's where I'd got those timings.

    Please visit this page for a schematics that i'd found on the net:http://comet.columbia.edu/msl/2000class/z80phone/ Scroll down to the images with the DAC and the LM386.

    There are some things which I don't understand:
    -are the timings really affected by the voltage we apply to the DAC?
    - why is iout (pin11) of the DAC attached to a diode(LM336)?
    -why is the Vref then attached to LM386?

    If the schematics actually worked, can we replace the diode with a simpler circuit, such that i don't have to include one more chip?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    With respect to your timing question - the DAC is CMOS, and switches faster with higher levels of Vcc.

    The output is connected to an operational amplifier (the LM336).

    You still have to come up with a means of controlling the D to A converter.

    For audio use, the output will sound very poor & choppy. It's an 8 bit DAC - most audio DAC's are more like 20 - 22 bits.
     
  5. oddabe83

    Thread Starter New Member

    Mar 31, 2007
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    Thanks for the fast reply.
    To use the 80188 to control the DAC, I'll connect the WR or RD lines and PCS lines directly to the DAC. Thus, whenever i use a IN or OUT command, these lines will be asserted and the DAC can be controlled this way.

    My doubts:
    - Why connect an op amp (LM336) to the output of the DAC when the speaker isn't connected to it at all in the first place?
    - Why connect the LM386 to the Vref instead? i though Vref should be fixed, as it should be a voltage reference for all signals?

    Do you think I'll need to use 9V in this case to power the DAC? Anyhow, 325ns is rather slow don't you think? That is equivalent to a approximately 2++Mhz processor??! Thus, I feel that the AC characteristics written on the datasheet is not really the timings of the DAC?

    Please advice.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    I have no idea of your background, but you seem to have missed the idea of interfaces. There is a need to have some hardware between the processor and the outside world. In this case, it can make the difference in the processor data bus speed and the relatively slow response of the DAC invisible. That's how a 3 GHz processor can use the same DAC with no problems. Consider how likely it is that the manufacturer of the DAC is unaware of exactly how it works.

    Usually, there is some means of selecting the external device by means of the address bus. You might look up the subject of interfacing, and see if there are IC's available for the purpose - the device will probably be either a VIA (versatile interface adaptor) or PIA (parallel interface adaptor). Using such a device will give you the necessary control lines to and from the DAC.
     
  7. dionaide

    New Member

    Jul 30, 2007
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    thanks for sharing your knowledge...
     
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