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MAX3421E Programming Guide
SOFKAENAB
Meaning:
Enable automatic generation of full-speed SOF packets or low-speed Keep-Alive
pulses.
Mode:
Host only
The CPU sets and clears this bit.
Programming Notes
When the CPU sets SOFKAENAB = 1, the SIE automatically generates 1-millisecond frame
markers. If the bit LOWSPEED = 0, the SIE generates SOF packets. If LOWSPEED = 1, the SIE
generates keep-alive pulses.
The SOF or KA pulses start after the SOFKAENAB bit has asserted for 1 millisecond. If the
CPU sets SOFKAENAB = 0 while the SIE is generating a frame marker, the SIE completes the
signaling before shutting off the frame markers.
SOF Packets
Every millisecond the SIE sends the SOF PID, the contents of an internal 11-bit frame counter,
and a CRC5 value. After sending the packet it increments the frame counter and asserts the
FRAMEIRQ. The SIE does not update any HRSLT bits in the HRSL register, thus giving the
CPU all the time needed to read the result of the last transfer.
The power-on default value of the frame counter is zero. The CPU can reset the frame counter at
any time by setting the bit FRMRST = 1 (page
).
Keep-Alive Pulses
When operating as a low-speed host and when SOFKAENAB = 1, the SIE generates a keep-alive
pulse that consists of one low-speed EOP every millisecond