DMTimer
The OCP clock domain synchronous registers TIDR, TIOCP_CFG, TISTAT, IRQSTATUS,
IRQSTATUS_RAW, IRQENABLE_SET, IRQENABLE_CLR, IRQWAKEEN, TWPS and TSICR are not
affected by the posted/non-posted mode selection; the write/read operation is effective and acknowledged
(command accepted) after one OCP clock cycle from the command assertion.
NOTE:
Only Posted Mode is recommended on this device because of the
PICLKTIMER < PICLKOCP/4 internal clocking requirement.
The Non-Posted Mode can be also used when freq (timer) < freq (OCP)/4, but it is recommended to use
the Posted Mode. because Non-Posted Mode will delay the command accept and the transaction latency
will be as described in the below chapters. Posted mode offers an OCP interface latency improvement
when freq(timer) < freq (OCP)/4.
20.1.3.4 Write Registers Access
20.1.3.4.1 Write Posted
This mode can be used only if the functional frequency range is freq (timer) < freq (OCP)/4.
This mode is used if TSICR (POSTED bit) is set to 1 in the timer control register.
This mode uses a posted-write scheme to update any internal register. The write transaction is
immediately acknowledged on the OCP interface, although the effective write operation will occur later,
due to a resynchronisation in the timer clock domain. This has the advantage of not stalling either the
interconnect system, or the CPU that requested the write transaction. For each register, a status bit is
provided, that is set if there is a pending write access to this register.
In this mode, it is mandatory that the CPU checks the status bit prior to any write access. In case a write is
attempted to a register with a previous access pending, the previous access is discarded without notice
(this can lead to unexpected results also).
There is one status bit per register, accessible in the Timer Write Posted Status Register. When the timer
module operates in this mode, there is an automatic sampling of the current timer counter value, in an
OCP-synchronized capture register. Consequently, any read access to the timer counter register does not
add any re-synchronization latency; the current value is always available.
A register read following a write posted register (on the same register) is not insured to read the previous
write value if the write posted process is not completed. Software synchronization should be used to avoid
non-coherent read.
The drawback of this automatic update mechanism is that it assumes a given relationship between the
OCP interface frequency and the timer functional frequency.
This posted period is defined as the interval between the posted write access request and the reset of the
posted bit in TWPS register, and can be quantified:
T (reset posted max.) = 3 OCP clock + 2.5 TIMER clock
The time when the write accomplishes is:
T (write accomplish) = 1 OCP clock + 2.5 TIMER clock
3564
Timers
SPRUH73H – October 2011 – Revised April 2013
Copyright © 2011–2013, Texas Instruments Incorporated