Computer Hardware User's Guide

Serial Ports
12-39 Peripherals
Figure 12–29. Exiting Fixed Continuous Mode Without Frame Sync, FSX Internal
CLKX
FSX
(internal)
DX
LOAD DXR SET XFSM RESET XFSM
A1 AN B1 BN C1 CN D1 DN E1 EN F1 FN
1st word 2nd word 3rd word 4th word 5th word
12.2.12.2 Variable Data-Rate Timing Operation
The following variations are included in variable data-rate timing operations.
Variable Burst Mode
In burst mode with variable data-rate timing, FSX/FSR pulse lasts for the
entire duration of transfer. With an internally generated FSX (see
Figure 12–30), transmission is initiated by loading DXR. In this mode there
is a delay of approximately 3.5 CLKX cycles (depending on CLKX and H1
freqency) from the time DXR is loaded until FSX occurs. With an external
FSX, the FSX pulse initiates the transfer and the 3.5-cycle delay effectively
becomes a setup requirement for loading DXR with respect to FSX. There-
fore, in this case, you must load DXR no later than four CLKX cycles before
FSX occurs. Once the XSR is loaded from the DXR, an XINT is generated.
Figure 12–30. Variable Burst Mode
DXR loaded XINT RINT
CLKX/R
FSR/FSX (external)
FSX (internal)
DX/DR
A1 AN
R/XVAREN = 1
R/XFSM = 0