Haswell C states raise PSU compatibility fears

May 1, 2013 // 10:20 a.m.

Tags: #c6 #c7 #energy-saving #enermax #haswell #haswell-compatibility #haswell-upgrade #intel #power-saving #power-supply #psu #upgrade #zero-load

New power efficiency features in Intel's upcoming Haswell processors, due to launch in June at the Computex event in Taipei, could mean they fail to work correctly with certain power supplies, it has been claimed.

Intel has made much about the low-power capabilities of Haswell, the architecture on which its upcoming fourth-generation Core processors are based - even claiming that Haswell-based laptops will be able to spend up to ten days in an internet-connected semi-standby mode from a single charge. Much of this efficiency comes from new processor states, C6 and C7, which drop the power consumption of a sleeping Haswell processor to a tenth that of its Ivy Bridge predecessors.

There's a problem, though: not all power supplies are designed to cope with running at such low current. For original equipment manufacturers (OEMs) building laptops, that's not a problem: they can just specify power supply circuitry happy running at very low loads. For enthusiasts hoping to upgrade existing Ivy Bridge or Sandy Bridge desktops to Haswell, however, it's bad news.

'Only few power supplies will be able to deliver stable voltages at such low loads,' claims Enermax, one of the few power supply companies to talk publicly on the issue. 'End users are therefore groping in the dark with no clues if their own power supply will be compatible with the new energy functions of Intel Haswell CPUs. PSU manufacturers usually do not state the possible minimum load of their products.'

The fact that Enermax is willing to highlight the problem should give you a hint as to what comes next: the company, which claims to be 'one of the technologically leading power supply manufacturers' in its press release on the matter, is looking to reassure its customers that as long as they have an Enermax power supply, they should be right as rain for a Haswell upgrade. 'All current high-end and mid-range models from Enermax are already prepared for the upcoming Intel processors,' the company claims. 'They are equipped with a DC-to-DC converter which enables the so-called ZERO Load Design. These power supplies will deliver rock-stable voltages even at 0W load.'

That design, which allows the power supply to operate correctly even when there is no current being demanded by the system, is something the company adopted back in 2008 with the launch of the Revolution85+, and has been using in its top-end models ever since. Not all Enermax power supplies get the required DC-to-DC circuitry, however: the Platimax 500W, 600W, 750W, 850W, 1000W, 1200W and 1500W models, Revolution 87+ 550W, 650W, 750W, 850W and 1000W, MaxRevo 1200W, 1350W, 1500W and Triathlor 385W, 450W and 550W all include the circuitry as standard, but other current Enermax PSUs do not - and thus are unlikely to be compatible with Haswell.

So far, no other manufacturers have publicly commented on the issue, which was first raised by VR-Zone late last week. It seems likely, however, that motherboard manufacturers will disable the new C6 and C7 states by default - circumventing the problem - rather than face support calls from angry customers wondering why their new motherboard and processor combination is failing to work on a known-good power supply.

For those who actively want the energy saving features of Haswell - something which is less important in a desktop than a laptop, despite rising energy costs - the message is clear: if you're building a new rig, choose your power supply carefully; if you're upgrading, budget for a new PSU just in case.