MC88915TFN55

The MC88915T Clock Driver utilizes phaselocked loop technology to lock its low skew outputs' frequency and phase onto an input reference clock. It is designed to provide clock distribution for high performance PC's and workstations. For a 3.3V version, see the MC88LV915T data sheet

The PLL allows the high current, low skew outputs to lock onto a single clock input and distribute it with essentially zero delay to multiple components on a board. The PLL also allows the MC88915T to multiply a low frequency input clock and distribute it locally at a higher (2X) system frequency. Multiple 88915's can lock onto a single reference clock, which is ideal for applications when a central system clock must be distributed synchronously to multiple boards (see Figure 7).

Five "Q" outputs (Q0Q4) are provided with less than 500 ps skew between their rising edges. The Q5 output is inverted (180° phase shift) from the "Q" outputs. The 2X_Q output runs at twice the "Q" output frequency, while the Q/2 runs at 1/2 the "Q" frequency.

The VCO is designed to run optimally between 20 MHz and the 2X_Q F^_max specification. The wiring diagrams in
Figure 5 detail the different feedback configurations which create specific input/output frequency relationships. Possible frequency ratios of the "Q" outputs to the SYNC input are 2:1, 1:1, and 1:2.

The FREQ_SEL pin provides one bit programmable divideby in the feedback path of the PLL. It selects between divideby1 and divideby2 of the VCO before its signal reaches the internal clock distribution section of the chip (see the block diagram on page 2). In most applications FREQ_SEL should be held high (÷1). If a low frequency reference clock input is used, holding FREQ_SEL low (÷2) will allow the VCO to run in its optimal range (>20MHz and >40MHz for the TFN133 version).

In normal phaselocked operation the PLL_EN pin is held high. Pulling the PLL_EN pin low disables the VCO and puts the 88915 in a static "test mode". In this mode there is no frequency limitation on the input clock, which is necessary for a low frequency board test environment. The second SYNC input can be used as a test clock input to further simplify boardlevel testing (see detailed description on page 11)

Pulling the OE /RST pin low puts the clock outputs 2X_Q, Q0Q4, Q5 and Q/2 into a high impedance state (3state). After the OE /RST pin goes back high Q0Q4, Q5 and Q/2 will be reset in the low state, with 2X_Q being the inverse of the selected SYNC input. Assuming PLL_EN is low, the outputs will remain reset until the 88915 sees a SYNC input pulse.

A lock indicator output (LOCK) will go high when the loop is in steadystate phase and frequency lock. The LOCK output will go low if phaselock is lost or when the PLL_EN pin is low. The LOCK output will go high no later than 10ms after the 88915 sees a SYNC signal and full 5V VCC.