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MV1820DPAS Scheda tecnica(PDF) 3 Page - Mitel Networks Corporation |
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MV1820DPAS Scheda tecnica(HTML) 3 Page - Mitel Networks Corporation |
3 / 9 page MV1820 4 CRYSTAL SPECIFICATION Parallel resonant fundamental frequency 27.750000MHz. AT cut. Nominal load capacitance 20pF. Tolerance at -10 °C to 60°C ± 50ppm. Equivalent series resistance <20 Ω. Tolerance overall ± 100ppm. FUNCTIONAL DESCRIPTION The video signal is sliced to produce data and synchronising signals. Timing circuits monitor the sync signal to enable the MV1820 to lock onto the broadcast signal. A timing window, for the Vertical Blanking Interval (VBI) lines 6 - 22 and 318 - 335, is established to enable the acquisition circuit to monitor the sliced data signal for valid teletext data. The framing code is checked for valid World System Teletext (WST) data. Magazine, packet and designation code bytes are checked and valid Broadcast Service Data Packets (BSDP) format two type only are accepted. These are known as packet 8/30. Format two is signalled by byte six, data bit two being set high and bits 3 and 4 set low. Bytes 13 to 25 inclusive are Hamming decoded (8,4) and stored in seven registers each of eight bits. If the complete message is correctly received with no uncorrectable Hamming errors, an interrupt to the microprocessor is signalled by the DAV (bar) pin going low. At the same time the data is transferred to a second bank of registers, reorganised with original numbered bytes 14, 15, 24, 25 and 13 placed after byte 23, to be read out on the I2C bus when so requested. Subsequent valid messages will continue to be transferred to the output registers overwriting any existing data. In this way the output registers always contain the latest PDC message. The MV1820 is configured as an I2C bus slave transmitter with a selectable address. The I2C bus address is 0010 0001 (20 + 1 hex) with the address select (AS) pin set high, or 0010 0011 (22 + 1 hex) with the AS pin set low. The read bit (LSB) must always be set, it is not possible to write to the MV1820. On recognising its address, the MV1820 will send an acknowledge and then transmit on the SDA line the first byte from the output registers (decoded byte 16 and 17) most significant bit (MSB) first. It will then monitor the SDA line for an acknowledge from the microprocessor. If the microprocessor does NOT send an acknowledge, the MV1820 will release the data line to allow the microprocessor to send a stop condition. If the microprocessor does send an acknowledge, the following bytes of the message will be output provided each byte is acknowledged. The final data will be byte 13 followed by the four ‘1’s. When readout is complete, the DAV (bar) pin is reset high and the output registers are all set high. If the microprocessor continues to send clocks on the SCL line, the MV1820 will output FF bytes on the SDA line. Also, if the MV1820 is re- addressed before another PDC message is received, the MV1820 will output FF bytes on the SDA line. The microprocessor can prematurely stop the message by NOT sending an Acknowledge followed by a STOP condition after any byte has been sent by the MV1820. The registers will then be reset to FF bytes and the DAV pin will be reset high. To prevent any corruption of the data in the output registers during I2C bus activity, valid PDC messages are held in the incoming registers until I2C bus activity ceases. Here they may be overwritten by new PDC messages until the I2C bus activity ceases and they can then be transferred to the output registers. System clock is provided by an on - chip 27.75MHz oscillator together with an external parallel resonant fundamental frequency AT cut crystal. Following a reset, RESET pulled low, the output I2C bus registers will contain FF bytes and the DAV pin will be set high. When the power supply is removed, the I2C bus will not be clamped to ground, leaving it free for other I2C bus traffic. Fig.3 Typical application diagram |
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