NEC Network Controller uPD98502 - Manual (Page 333)

CHAPTER 6 USB CONTROLLER
Preliminary User’s Manual S15543EJ1V0UM 333
Figure 6-3. Mailbox Configuration
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U_TMSA(U_RMSA)
U_TMWA(U_RMWA)
U_TMBA(U_RMBA)
U_TMRA(U_RMRA)
When USB Controller writes an indication, the write pointer (U_TMWA or U_RMWA) is incremented. Every time
that USB Controller writes an indication, it also sets the transmit/receive finish bit of the corresponding EndPoint and,
issues an interrupt if it is not masked.
The write pointer is forced to jump to the start address (U_TMSA or U_RMSA) when it reaches the bottom address
(U_TMBA or U_RMBA). USB Controller uses the read pointer (U_TMRA or U_RMRA) to prevent the overwriting of
those indications that the V R 4120A has not yet read out. The read pointer (U_TMRA or U_RMRA) is managed by the
V R 4120A. Each time the V R 4120A reads an indication from a MailBox, it writes the address to be read next time into
the read pointer register (U_TMRA or U_RMRA).
When both the write pointer (U_TMWA or U_RMWA) and read pointer (U_TMRA or U_RMRA) point to the same
address, USB Controller sets the TMF bit (transmit MailBox full) or RMF bit (receive MailBox full) of the USB General
Status Register 1 to indicate the MailBox full state and issues an interrupt if it is not masked.
In the MailBox full status, USB Controller will not issue the next indication. The V R 4120A must read an indication
from the full MailBox and update the read pointer (U_TMRA or U_RMRA).