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MC68194 Datasheet(PDF) 8 Page - ON Semiconductor |
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MC68194 Datasheet(HTML) 8 Page - ON Semiconductor |
8 / 24 page MC68194 http://onsemi.com 8 SECTION 3 TRANSMITTER 3.1 OVERVIEW The transmitter function includes the serial interface decoder, transmit modulator, transmit buffer, jabber inhibit, and clock generator. (Although the clock generator is not used exclusively by the transmit function, the generator will be discussed here.) The MC68194 receives request channel symbols on the TXSYMX pins which are synchronized to TXCLK. As is described in the Serial Interface discussion, MAC transmit symbols are input serially (CBM in MAC mode), decoded, and used to modulate an output signal. The Serial Interface Decoder is used both for MAC mode to decode data transmit commands (symbols) and management mode to decode management commands. The decoded transmit commands or symbols are used by the Transmit Modulator to generate phase–coherent signaling as discussed in the CBM General Description. The transmit buffer receives the modulated signal and drives differential output signals. The clock generator provides TXCLK and internal clocks of 2 times (2X) and 4 times (4X) TXCLK. The 4X clock is actually the oscillator frequency. These clocks are used to receive the TX symbols and generate the modulated signal. 3.2 TRANSMIT BUFFER The modulated transmit data stream drives the TXOUT and TXOUT* pins of the MC68194. These pins are complementary outputs with closely matched edge transitions. This is useful in helping meet the IEEE 802.4 carrierband requirement for a transmit jitter of less than "1% of the data rate. TXOUT and TXOUT* are generally used to drive a differential amplifier which is used to achieve the necessary output level at the cable and meet the rise/fall time window (or “eye” pattern) of the IEEE 802.4. A third output called TXDIS is available to gate the amplifier circuitry on or off. The TXTOUT and TXTOUT* have ECL levels referenced to VCC (Figure 3–1). Levels are typically 4.11 V for a high and 3.25 for a low. Pulldown resistors are required with the outputs specified to drive a maximum load of 220 Ω to ground reference. Operation of the transmit outputs is controlled in the following manner: 1. Management mode — The TX outputs are always disabled while the CBM is in management mode. When leaving management mode the TX outputs remain disabled if a RESET command has been issued and an ENABLE TRANSMITTER and DISABLE LOOPBACK commands have not been issued. Resetting the CBM enables internal loopback and disables the transmitter. 2. MAC (data) mode — After leaving management mode, the CBM can function in internal loopback (for test) with the transmitter disabled, out of loopback with transmitter disabled (receive only), or in standard data mode with the TX outputs controlled by the modulator. Figure 3–1. Transmitter Outputs RP TXOUT RP TX AMP VCC–TXOUT TXDIS TXOUT* 3. Jabber inhibit activated — If the jabber inhibit fires, it forces the CBM into management mode and disables the TX outputs. This condition can only be cleared by a reset condition. The TXDIS output is an open collector switched current source. TXDIS sinks a nominal 0.5 mA when the TXOUT/TXOUT* outputs are enabled. TXDIS is off or high impedance when the transmitter is disabled. The signaling on the TX outputs and TXDIS is shown in Figure 3–2. The “off” or silence condition is both TXOUT outputs high and TXDIS also high. The figure shows an example of the signal pattern for both leaving and entering a silence condition. Figure 3–2. Transmitter Output Signaling SILENCE OFF OFF TXDIS TXOUT* 0 TXOUT 1 1 ND1 ND2 1 |
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