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and allows the CPUs asserting TLSB_LOCKOUT to complete their bus ac-
cess without interference.
TLSB_LOCKOUT is asserted for one cycle then deasserted for one cycle.
This two-cycle sequence may be repeated until the device is ready to
deassert TLSB_LOCKOUT. Multiple devices may assert this signal in any
of these two-cycle sequences.
Devices must disregard the value of TLSB_LOCKOUT received during the
second of each two-cycle sequence, as it is Unpredictable. An assertion of
TLSB_LOCKOUT should be converted internally to look like a two-cycle
assertion and ignore the actual value received in the second cycle. This en-
tire sequence repeats every two cycles until it is received deasserted.
TLSB_LOCKOUT must be initially asserted in the cycle following an arbi-
tration cycle, that is, at the same time that a command and address are
valid. Continued assertions must follow in successive two-cycle sequences,
independent of any additional arbitration cycles. Before asserting
TLSB_LOCKOUT, a device must check whether the signal is already being
asserted by another node and synchronize with the existing two-cycle se-
quence.
2.3 CSR Addressing
Two types of control and status registers (CSRs) can be accessed on the
TLSB. Local CSRs are implemented in the TLSB nodes in the system. Re-
mote CSRs exist on I/O devices connected to I/O buses. They are accessed
through I/O nodes in the system.
There are two ways to access remote CSRs:
Mailbox access allows full software control of the communication with the
remote I/O device. Window space CSR access maps physical addresses to
registers in a remote I/O device. Window space access eliminates the need
for software to manipulate mailboxes and allows a single I/O space refer-
ence to read or write a remote CSR.
Two command codes are used on the system for CSR accesses: CSR write
and CSR read. The use of these special command codes allows full use of
the address and bank number fields on the address bus.
Figure 2-4 shows the address bit mapping of the TLSB CSRs.
• Mailbox
• Window space
TLSB Bus 2-25
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