
Setting the Receive Frame-Sync Polarity
7-27
Receiver Configuration
SPRU592E
FSRP, FSXP, CLKRP, and CLKXP in the pin control register (PCR) configure
the polarities of the FSR, FSX, CLKR, and CLKX signals, respectively. All
frame-sync signals (internal FSR, internal FSX) that are internal to the serial
port are active high. If the serial port is configured for external frame
synchronization (FSR/FSX are inputs to McBSP), and FSRP = FSXP = 1, the
external active-low frame-sync signals are inverted before being sent to the
receiver (internal FSR) and transmitter (internal FSX). Similarly, if internal
synchronization (FSR/FSX are output pins and GSYNC = 0) is selected, the
internal active-high frame-sync signals are inverted, if the polarity bit
FS(R/X)P = 1, before being sent to the FS(R/X) pin.
On the transmit side, the transmit clock polarity bit, CLKXP, sets the edge used
to shift and clock out transmit data. Note that data is always transmitted on the
rising edge of internal CLKX. If CLKXP = 1, and external clocking is selected
(CLKXM = 0 and CLKX is an input), the external falling-edge triggered input
clock on CLKX is inverted to a rising-edge triggered clock before being sent
to the transmitter. If CLKXP = 1, and internal clocking selected (CLKXM = 1
and CLKX is an output pin), the internal (rising-edge triggered) clock, internal
CLKX, is inverted before being sent out on the CLKX pin.
Similarly, the receiver can reliably sample data that is clocked with a rising
edge clock (by the transmitter). The receive clock polarity bit, CLKRP, sets the
edge used to sample received data. Note that the receive data is always
sampled on the falling edge of internal CLKR. Therefore, if CLKRP = 1 and
external clocking is selected (CLKRM = 0 and CLKR is an input pin), the
external rising-edge triggered input clock on CLKR is inverted to a falling-edge
triggered clock before being sent to the receiver. If CLKRP = 1, and internal
clocking is selected (CLKRM = 1), the internal falling-edge triggered clock is
inverted to a rising-edge triggered clock before being sent out on the CLKR pin.
Note that CLKRP = CLKXP in a system where the same clock (internal or
external) is used to clock the receiver and transmitter. The receiver uses the
opposite edge as the transmitter to ensure valid setup and hold of data around
this edge. Figure 7
18 shows how data clocked by an external serial device
using a rising edge can be sampled by the McBSP receiver on the falling edge
of the same clock.
Содержание TMS320VC5509
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