Texas Instruments TMS320C3x - Manual (Page 416)

Serial Ports
12-37 Peripherals
Figure 12–27. Fixed Standard Mode With Back-to-Back Frame Sync
A1 AN B1 BN C1
DXR loaded
with A
XINT
DXR loaded
with B
XINT
RINT XINT
RINT
CLKX/R
FSX (Internal)
FSR/FSX (External)
DR/DX
Load DXR with C
read DRR
Load DXR with
Dread DRR
R/XVAREN = 0
R/XFSM = 0
For receive operations and with externally generated FSX, once transfers
have begun, frame sync pulses are required only during the last bit trans-
ferred to initiate another contiguous transfer. Otherwise, frame sync inputs
are ignored. Continuous transfers occur if the frame sync is held high. With
an internally generated FSX, there is a delay of approximately 2.5 CLKX
cycles from the time DXR is loaded until FSX occurs. This delay occurs
each time DXR is loaded; therefore, during continuous transmission, the
instruction that loads DXR must be executed by the
N–3 bit for an N-bit
transmission. Since delays due to pipelining vary, you should incorporate
a conservative margin of safety in allowing for this delay.
Once the process begins, an XINT and an RINT are generated at the begin-
ning of each transfer. The XINT indicates that the XSR has been loaded from
DXR and can be used to cause DXR to be reloaded. To maintain continuous
transmission in fixed rate mode with frame sync, especially with an internally
generated FSX, DXR must be reloaded early in the ongoing transfer.
The RINT indicates that a full word has been received and transferred into
the DRR; RINT indicates an appropriate time to read DRR.
Continuous transfers are terminated by discontinuing frame sync pulses
or, in the case of an internally-generated FSX, not reloading DXR.
- Fixed Continuous Mode
You can accomplish continuous serial-port transfers, without the use of
frame sync pulses, if R/XFSM is set to 1. In this mode, operation of the serial
port is similar to continuous operation with frame sync, except that a frame
sync pulse is involved only in the first word transferred, and no further frame
sync pulses are used. Following the first word transferred (see
Figure 12–28), no internal frame sync pulses are generated, and frame