Rainbow Electronics DS3170 - Manual (Page 66)

DS3170 DS3/E3 Single-Chip Transceiver
66 of 233
9.4 Global Resources
9.4.1 Clock Rate Adapter (CLAD)
The clock rate adapter is composed of a PLL block to create the internal clock which can be used for the transmit
clock and/or LIU reference clock from a clock input on the reference input (REFCLK) pin. The device needs one of
two (DS3 or E3) internal clock rates. The input reference clock frequency can be either 44.736, 34.368. 77.78,
51.84 or 19.44 MHz.
The receive LIU is supplied a reference clock from the CLAD. The receive LIU selects the clock frequency based
upon the mode the user selects via the FM bits. The CLAD output is also available as a transmit clock source if
selected via the
PORT.CR2 .CLADC register bit.
The user must supply at least one of the five rates (44.736, 34.368. 77.78, 51.84 or 19.4 MHz) to the REFCLK pin.
The CLAD[2:0] bits informs the PLL of the frequency applied to the pins. Selection of the clock applied to the LIU
and optionally the transmitter is controlled by the FM bits (located in
PORT.CR2 ). The CLAD allows maximum
flexibility to the user. The user may supply any of the five clock rates and use the CLAD to convert the rate to the
particular clock rate needed for his application.
The CLAD PLL is enabled when the CLAD input reference clock is different from the clock required for the framing
mode. The CLAD PLL is disabled and the CLAD output clock is connected directly to the CLAD input clock
(REFCLK) when the framing mode requires the same clock as the CLAD input reference clock.
Table 9-11. CLAD Clock Source Settings
CLAD[2:0] REFCLK (INPUT)
000 44.736 MHz
001 34.368 MHz
010 51.84 MHz
011 19.44 MHz
100 77.76 MHz
101 Undefined
11X Undefined
9.4.2 8 kHz Reference Generation
The global 8KREF signal is used to generate the one second reference signal by dividing it by 8000. This signal
can be derived from almost any clock source on the chip as well as the general purpose IO pin GPIO4. The port
8KREF signal can be sourced from the transmit or receive clocks. The minimum input frequency stability of the
8KREF input pin is +/- 500 ppm.
The global 8KREF signal can come from an external 8000 Hz reference connected to the GPIO4 general purpose
IO pin by setting the
GL.CR2. G8KIS bit. The global 8KREF signal can be output on the GPIO2 general purpose IO
pin when the GL.CR2 .G8KOS bit is set.
The global 8KREF signal can be derived from the CLAD PLL or pins or come from any of the port 8KREF signals
by clearing
GL.CR2. G8KIS bit and selecting the source using the GL.CR2. G8KRS[2:0] bits.
The port 8KREF signal can be derived from the transmit clock input pin or from the receive LIU or input clock pin.
The
PORT.CR3 .P8KRS[1:0] bits are used to select which source.
The 8KREF 8.000 kHz signal is a simple divisor of 44736 kHz (DS3 divided by 5592) or 33368 kHz (E3 divided by
4296). The correct divisor for the port 8KREF source is selected by the mode the port is configured for. The CLAD
clock chosen for the clock source selects the correct divisor for the global 8KREF. The 8KREF signal is only as
accurate as the clock source chosen to generate it.
Table 9-12 lists the selectable sources for global 8 kHz reference.