2.0 Circuit Description
2.3 Jitter Attenuator
2.3.1 Elastic Store
The elastic store size (RJAT or TJAT) is configurable using JSIZE[2:0] in the
JAT_CR. The elastic store sizes available are 8, 16, 32, 64, and 128 bits. The
32-bit elastic store depth is sufficient to meet jitter tolerance requirements in
cases where the jitter attenuator cutoff frequency is programmed at 6 Hz or below,
and when the selected clock reference is frequency-locked. The larger elastic
store depths allows greater accumulated phase offsets. For example, the 128-bit
depth can tolerate up to 64 bits of accumulated phase offset.
Since the elastic store is a fixed size, it can overflow and under-run. Overflow
occurs when the elastic store is full; under-run occurs when the elastic store is
empty. If either of these two conditions occurs, the Jitter Attenuator Elastic Store
Limit Error bit (JERR) in the Error Interrupt Status register [ISR5; addr 006] is
set. To determine if an overflow or under-run occurs, the Jitter Attenuator
Empty/Full bit (JMPTY) must be read from the Receive LIU Status register
[RSTAT; addr 021].
The elastic store is a circular buffer with independent read and write pointers.
The difference between the read and write pointers is the phase error (JPHASE)
between the input and output clocks of the jitter attenuator and is used to generate
JCLK. The read and write pointers are initialized using JCENTER in the
JAT_CR. JCENTER resets the write pointer and forces the elastic store read
pointer to 1 half of the programmed JSIZE. JCENTER also resets the JMPTY
status, so JMPTY must be read before JCENTER is written.
If JDIR is configured to put the jitter attenuator in the receive path, the write
pointer is driven by the Receive Clock (RXCLK), and the read pointer is driven
by the dejittered recovered clock (JCLK). The dejittered recovered clock output is
available on the RCKO pin if the output is enabled using RCKO_OE in the
Programmable Output Enable register [POE; addr 019]. The dejittering of the
recovered clock is done by the Clock Rate Adapter Block (CLAD). CLAD is
described later in this document.
If JDIR is configured to put the jitter attenuator in the transmit path, the write
pointer is driven by the Transmit Clock (TXCLK), and the read pointer is driven
by the dejittered transmit clock (JCLK). TXCLK can be slaved to four different
clock sources: Transmit Clock Input (TCKI), Receive Clock Output (RCKO),
Receive System Bus Clock Input (RSBCKI), or Clock Rate Adapter Output
(CLADO). The dejittered transmit clock is available on the TCKO pin when the
output is enabled using TCKO_OE in POE.
The receive LIU input clock and data jitter tolerance meets TR 62411-1990, as
The JAT jitter transfer function meets TR 62411-1990, as defined in
Fully Integrated T1/E1 Framer and Line Interface
2-7. The JAT input jitter tolerance is illustrated in