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    參數(shù)資料
    型號: XC5202-5PG223C
    廠商: Xilinx, Inc.
    英文描述: Field Programmable Gate Arrays
    中文描述: 現(xiàn)場可編程門陣列
    文件頁數(shù): 28/73頁
    文件大小: 598K
    代理商: XC5202-5PG223C
    R
    XC5200 Series Field Programmable Gate Arrays
    7-110
    November 5, 1998 (Version 5.2)
    Configuration
    The length counter begins counting immediately upon entry
    into the configuration state. In slave-mode operation it is
    important to wait at least two cycles of the internal 1-MHz
    clock oscillator after INIT is recognized before toggling
    CCLK and feeding the serial bitstream. Configuration will
    not begin until the internal configuration logic reset is
    released, which happens two cycles after INIT goes High.
    A master device’s configuration is delayed from 32 to 256
    μ
    s to ensure proper operation with any slave devices driven
    by the master device.
    The 0010 preamble code, included for all modes except
    Express mode, indicates that the following 24 bits repre-
    sent the length count. The length count is the total number
    of configuration clocks needed to load the complete config-
    uration data. (Four additional configuration clocks are
    required to complete the configuration process, as dis-
    cussed below.) After the preamble and the length count
    have been passed through to all devices in the daisy chain,
    DOUT is held High to prevent frame start bits from reaching
    any daisy-chained devices. In Express mode, the length
    count bits are ignored, and DOUT is held Low, to disable
    the next device in the pseudo daisy chain.
    A specific configuration bit, early in the first frame of a mas-
    ter device, controls the configuration-clock rate and can
    increase it by a factor of eight. Therefore, if a fast configu-
    ration clock is selected by the bitstream, the slower clock
    rate is used until this configuration bit is detected.
    Each frame has a start field followed by the frame-configu-
    ration data bits and a frame error field. If a frame data error
    is detected, the FPGA halts loading, and signals the error
    by pulling the open-drain INIT pin Low. After all configura-
    tion frames have been loaded into an FPGA, DOUT again
    follows the input data so that the remaining data is passed
    on to the next device. In Express mode, when the first
    device is fully programmed, DOUT goes High to enable the
    next device in the chain.
    Delaying Configuration After Power-Up
    To delay master mode configuration after power-up, pull
    the bidirectional INIT pin Low, using an open-collector
    (open-drain) driver. (See
    Figure 12
    .)
    Using an open-collector or open-drain driver to hold INIT
    Low before the beginning of master mode configuration
    causes the FPGA to wait after completing the configuration
    memory clear operation. When INIT is no longer held Low
    externally, the device determines its configuration mode by
    capturing its mode pins, and is ready to start the configura-
    tion process. A master device waits up to an additional 250
    μ
    s to make sure that any slaves in the optional daisy chain
    have seen that INIT is High.
    Start-Up
    Start-up is the transition from the configuration process to
    the intended user operation. This transition involves a
    change from one clock source to another, and a change
    from interfacing parallel or serial configuration data where
    most outputs are 3-stated, to normal operation with I/O pins
    active in the user-system. Start-up must make sure that
    the user-logic ‘wakes up’ gracefully, that the outputs
    become active without causing contention with the configu-
    ration signals, and that the internal flip-flops are released
    from the global Reset at the right time.
    Figure 25
    describes start-up timing for the three Xilinx fam-
    ilies in detail. Express mode configuration always uses
    either CCLK_SYNC or UCLK_SYNC timing, the other con-
    figuration modes can use any of the four timing sequences.
    To access the internal start-up signals, place the STARTUP
    library symbol.
    Start-up Timing
    Different FPGA families have different start-up sequences.
    The XC2000 family goes through a fixed sequence. DONE
    goes High and the internal global Reset is de-activated one
    CCLK period after the I/O become active.
    The XC3000A family offers some flexibility. DONE can be
    programmed to go High one CCLK period before or after
    the I/O become active. Independent of DONE, the internal
    global Reset is de-activated one CCLK period before or
    after the I/O become active.
    The XC4000/XC5200 Series offers additional flexibility.
    The three events — DONE going High, the internal Reset
    being de-activated, and the user I/O going active — can all
    occur in any arbitrary sequence. Each of them can occur
    one CCLK period before or after, or simultaneous with, any
    of the others. This relative timing is selected by means of
    software options in the bitstream generation software.
    The default option, and the most practical one, is for DONE
    to go High first, disconnecting the configuration data source
    and avoiding any contention when the I/Os become active
    one clock later. Reset is then released another clock period
    later to make sure that user-operation starts from stable
    internal conditions. This is the most common sequence,
    shown with heavy lines in
    Figure 25
    , but the designer can
    modify it to meet particular requirements.
    Normally, the start-up sequence is controlled by the internal
    device oscillator output (CCLK), which is asynchronous to
    the system clock.
    XC4000/XC5200 Series offers another start-up clocking
    option, UCLK_NOSYNC. The three events described
    above need not be triggered by CCLK. They can, as a con-
    figuration option, be triggered by a user clock. This means
    that the device can wake up in synchronism with the user
    system.
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