Architecture
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SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Multichannel Buffered Serial Port (McBSP)
25.2.10 SPI Operation Using the Clock Stop Mode
The McBSP on this device does not support the SPI protocol.
25.2.11 Resetting the Serial Port: RRST, XRST, GRST, and RESET
Device reset or McBSP reset: When the McBSP is reset by device reset or McBSP reset, the state
machine is reset to its initial state. All counters and status bits are reset. This includes the receive status
bits RFULL, RRDY, and RSYNCERR, and the transmit status bits XEMPTY, XRDY, and XSYNCERR in
the serial port control register (SPCR).
The serial port can be reset in the following two ways:
•
Device reset (RESET pin is low) places the receiver, the transmitter, and the sample rate generator in
reset. When the device reset is removed (RESET = 1), FRST = GRST = RRST = XRST = 0 in SPCR,
keeping the entire serial port in the reset state.
•
The serial port transmitter and receiver can be independently reset by the XRST and RRST bits in
SPCR. The sample rate generator is reset by the GRST bit in SPCR.
shows the state of the McBSP pins when the serial port is reset by these methods.
Table 25-18. Reset State of McBSP Pins
Pin
Direction
Device Reset ( RESET = 0)
McBSP Reset
Receiver Reset (RRST = 0 and GRST = 1)
DR
I
Input
Input
CLKR
I/O/Z
Input
Known state if input; CLKR if output
FSR
I/O/Z
Input
Known state if input; FSRP(inactive state) if output
CLKS
I
Input
Input
Transmitter Reset (XRST = 0 and GRST = 1)
DX
O/Z
High impedance
High impedance
CLKX
I/O/Z
Input
Known state if input; CLKX if output
FSX
I/O/Z
Input
Known state if input; FSXP(inactive state) if output
CLKS
I
Input
Input
25.2.11.1 Software Reset Considerations
McBSP reset:
When the receiver and transmitter reset bits, RRST and XRST in SPCR, are written with 0,
the respective portions of the McBSP are reset and activity in the corresponding section stops. All input-
only pins, such as DR, and all other pins that are configured as inputs are in a known state. FS(R/X) is
driven to its inactive state (same as its polarity bit, FS(R/X)P) if it is an output. If CLK(R/X) are
programmed as outputs, they are driven by CLKG, provided that GRST = 1. The DX pin is in the high-
impedance state when the transmitter is reset. During normal operation, the sample rate generator can be
reset by writing a 0 to GRST. The sample rate generator should only be reset when not being used by the
transmitter or the receiver. In this case, the internal sample rate generator clock CLKG, and its frame sync
signal (FSG) is driven inactive (low). When the sample rate generator is not in the reset state (GRST = 1),
FSR and FSX are in an inactive state when RRST = 0 and XRST = 0, respectively, even if they are
outputs driven by FSG. This ensures that when only one portion of the McBSP is in reset, the other
portion can continue operation when FRST = 1 and frame sync is driven by FSG.
Sample-rate generator reset:
As mentioned previously, the sample rate generator is reset when the
device is reset or when its reset bit, GRST in SPCR, is written with 0.
Emulator software reset:
In the event of an emulator software reset initiated from the CPU, the McBSP
register values are reset to their default values.