User Manual
BCM1250/BCM1125/BCM1125H
10/21/02
B r o a d c o m C o r p o r a t i o n
Document
1250_1125-UM100CB-R
Section 10: Serial Interfaces Page
339
I
NPUT
L
INE
I
NTERFACE
In the receive direction, data on pin DIN is sampled either by an external clock signal, supplied on the
CIN_RCLKIN pin, or by the internal baud rate generator. The internal baud clock can be made available
externally on the COUT pin by setting the appropriate bit in the
duart_opcr
register (
in the UART control block. Regardless of source, the polarity of the sampling clock edge is programmable. The
clock configuration is done in the
ser_clk
register.
There are three ways that data bits are qualified to determine if they should be accepted and sent as part of
the bit stream to the protocol engine.
1
Gapped Clock
: If the external device is supplying the clock on CIN_RCLKIN, it can omit clock pulses. Since
the BCM1250 never receives a clock pulse this method can always be used to suppress the reception of bits.
2
External Enable
: Regardless of clock source, the RIN pin can be supplied with an externally generated enable
signal. This can qualify the current data bit or may be delayed by 1, 2 or 3 clocks.
3
Internal Sequencer
: Regardless of clock source, but exclusive with (2), the enable signal can be generated
by an internal sequencer, which is itself synchronized to the data stream by a pulse on RIN. The sequencer
can also provide a strobe on the RSTROBE output.
The bit stream delivered to the protocol engine consists of the bits sampled during the enabled bit times; clock
edges occurring during disabled bit times are suppressed and not seen by the protocol engine.
Input Using an External Enable
An external enable signal can be provided on the RIN pin. This can be configured to be active high or low, and
indicates valid data when active. The Data (on DIN) and enable (on RIN) are latched on the same edge of the
clock. In the simplest case the enable qualifies the data bit that is latched at the same time, but the enable can
also be delayed to qualify the data 1, 2 or 3 clocks later.
Figure 68: Example Reception Using RIN as Active High Enable (sampling on the falling clock edge)
The example in
shows data and an active high enable being latched on the falling edge of the clock.
The enable becomes inactive during cycle 0. If the enable delay is set to 0 then the data bit at time 0 will be
ignored and the others will be accepted. If the enable delay is 3, the data bit at time 3 is skipped, while data
bits at times 4, 5 and 6 are accepted (the example does not show the falling edges of cycles -3, -2, and -1 so
it is not possible to determine from this figure if the data at bit times 0, 1, or 2 will be accepted).
Clock
Time:
DIN
RIN/En
0
1
2
3