W97AH6KB datasheet(49/127 Pages) WINBOND

49 / 127 page

W97AH6KB / W97AH2KB

Publication Release Date: May 15, 2014

Revision: A01-001

- 49 -

In contrast to JESD79 and JESD79-2 and JESD79-3 compliant SDRAM devices, LPDDR2-S4 devices allow

significant flexibiliy in scheduling REFRESH commands, as long as the boundary conditions above are met. In the

most straight forward case a REFRESH command should be scheduled every tREFI. In this case Self-Refresh may

be entered at any time.

The users may choose to deviate from this regular refresh pattern e.g., to enable a period where no refreshes are

required. As an example, using a 1Gb LPDDR2-S4 device, the user can choose to issue a refresh burst of 4096

REFRESH commands with the maximum allowable rate (limited by tREFBW) followed by a long time without any

REFRESH commands, until the refresh window is complete, then repeating this sequence. The achieveable time

without REFRESH commands is given by tREFW - (R / 8) * tREFBW = tREFW - R * 4 * tRFCab.@ TCASE

≤ 85°C this can

be up to 32 mS - 4096 * 4 * 130 nS

≈ 30 mS.

While both - the regular and the burst/pause - patterns can satisfy the refresh requirements per rolling refresh

interval, if they are repeated in every subsequent 32 mS window, extreme care must be taken when transitioning

from one pattern to another to satisfy the refresh requirement in every rolling refresh window during the transition.

Figure of 7.4.16.3 shows an example of an allowable transition from a burst pattern to a regular, distributed pattern. If

this transition happens directly after the burst refresh phase, all rolling tREFW interval will have at least the required

number of refreshes.

Figure of 7.4.16.4 shows an example of a non-allowable transition. In this case the regular refresh pattern starts after

the completion of the pause-phase of the burst/pause refresh pattern. For several rolling tREFW intervals the

minimmun number of REFRESH commands is not satisfied.

The understanding of the pattern transition is extremly relevant (even if in normal operation only one pattern is

employed), as in Self-Refresh-Mode a regular, distributed refresh pattern has to be assumed, which is reflected in

the equation for R* above. Therefore it is recommended to enter Self-Refresh-Mode ONLY directly after the burst-

phase of a burst/pause refresh pattern as indicated in figure of 7.4.16.5 and begin with the burst phase upon exit

from Self-Refresh.

W97AH6KB датащи(PDF) 49 Page - Winbond