15.8.16 Set the Transmit Frame-Synchronization Polarity
Table 15-64. Register Bit Used to Set Transmit Frame-Synchronization Polarity
Register
Bit
Name
Function
Type
Reset
Value
PCR
3
FSXP
Transmit frame-synchronization polarity
R/W
0
FSXP = 0
Frame-synchronization pulse FSX is active high.
FSXP = 1
Frame-synchronization pulse FSX is active low.
15.8.16.1 Frame Synchronization Pulses, Clock Signals, and Their Polarities
Transmit frame-synchronization pulses can be generated internally by the sample rate generator (see
mode bit, FSXM, in PCR. FSX is also affected by the FSGM bit in SRGR2. For information about the effects of
FSXM and FSGM, see
). Similarly, transmit clocks can be selected to be inputs or outputs by
programming the mode bit, CLKXM, in the PCR (see
When FSR and FSX are inputs (FSXM = FSRM= 0, external frame-synchronization pulses), the McBSP detects
them on the internal falling edge of clock, internal MCLKR, and internal CLKX, respectively. The receive data
arriving at the DR pin is also sampled on the falling edge of internal MCLKR. These internal clock signals are
either derived from external source via CLK(R/X) pins or driven by the sample rate generator clock (CLKG)
internal to the McBSP.
When FSR and FSX are outputs, implying that they are driven by the sample rate generator, they are generated
(transition to their active state) on the rising edge of internal clock, CLK(R/X). Similarly, data on the DX pin is
output on the rising edge of internal CLKX.
FSRP, FSXP, CLKRP, and CLKXP in the pin control register (PCR) configure the polarities of the FSR, FSX,
MCLKR, and CLKX signals, respectively. All frame-synchronization 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-synchronization 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 and the polarity bit FS(R/X)P = 1, the
internal active-high frame-synchronization signals are inverted 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. 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 MCLKX 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. The receive data is always
sampled on the falling edge of internal MCLKR. Therefore, if CLKRP = 1 and external clocking is selected
(CLKRM = 0 and MCLKR is an input pin), the external rising-edge triggered input clock on MCLKR 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 MCLKR pin.
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.
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.
Multichannel Buffered Serial Port (McBSP)
950
TMS320x2806x Microcontrollers
SPRUH18I – JANUARY 2011 – REVISED JUNE 2022
Copyright © 2022 Texas Instruments Incorporated
Содержание TMS320 2806 Series
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