PC Card Voltage Requirements for Windows
This article provides information to help designers understand how the Microsoft Windows operating system applies voltage to a PC Card device. By following the guidelines presented in this article, PC Card vendors can avoid shipping devices that fail to configure and operate in certain socket controllers.
With the advent of support for 3.3V cards in CardBus controllers, some vendors are designing their legacy R2 ISA-based PC Card cards to use only 3.3V. However, R2 cards that support 3.3V-only cards will fail when inserted into older PCMCIA Interface Controller (PCIC) socket controllers. Because there is no standard method in PCIC controllers for determining the voltage requirements of a card, Windows requires that R2 cards which use 3.3V also support 5V.
With a properly designed R2 card, Windows will initially power up the card with 5V and read the CIS tuples. If the CIS tuples allow for 3.3V operation and if the version of Windows being used understands how to apply 3.3V through that specific socket controller, then Windows will switch the card to 3.3V mode.
Also, proper wiring of the Voltage Sense (VS) pins and coding of the Card Information Structure (CIS) in dual-voltage R2 cards is critical for two reasons:
1. | According to the PCMCIA March 1997 PC Card Standard, Page 8, Table 3-1, a dual-voltage R2 card should have the VS pins wired according to the second entry in the table (VS1=Ground, VS2=Open). However, the fourth entry in the table, for 3.3V-only cards, is identical to the second entry except for the 5V key versus the low-voltage key. Because CardBus controllers are unable to sense the keying of the inserted card, dual-voltage cards are seen as 3.3V only. |
2. | Some CardBus controllers have a protection mechanism that prevents powering the card at a voltage that is higher than that indicated by the VS pins. In this case, if the CIS tuples require 5V, the controller will refuse to power the card above 3.3V and the card will fail to configure. |
In some cases, the PCIC controller will properly power a 3.3V-only card when inserted. However, in most cases, an R2 3.3V-only card will be initially powered to 5V when inserted into a low-voltage socket that is connected to a PCIC controller that supports 3.3V. Some PC Card and PCIC controller combinations are not properly sensed through the VS pins. As a result, the Socketsv driver cannot trust the VS pins and must initially power the PC Card to 5V to access existing 5V PC Cards.
Because users might insert an R2 card into a socket that will be powered with 5V, Microsoft recommends that PC Card vendors design their cards to support dual voltage if they want to support 3.3V at all. Microsoft recommends that R2-version PC Card designs incorporate the following guidelines:
| • | If the R2 card supports 3.3V operation, it must also support 5V operation as well, making it a dual-voltage card. |
| • | The VS pins should always be wired to indicate the maximum voltage the card can accept. This means 5V in the case of a dual-voltage card; VS1=Open, VS2=Open. |
| • | The CIS tuples should have multiple configurable entries indicating first choice: Vcc = 3.3V; second choice: Vcc = 5V. |
When a dual-voltage R2 is designed with the above guidelines, the Windows PC Card driver will first apply the voltage indicated by the VS pins (5V) to read the CIS tuples. Later on, the driver will reduce the voltage to that specified by the CIS tuple (3.3V), provided the socket controller can be programmed into 3.3V mode. With this configuration, your card will work under Windows on all CardBus and PCIC controllers.
Call to action for PC Card voltage requirements:
| • | Design your R2-version PC Cards to support dual voltage if you want to accommodate 3.3V operation. |
| • | Wire the VS pins on your card and code your CIS tuple to be compatible with all CardBus and PCIC controllers. |
| • | Test your design in a variety of CardBus and PCIC controllers under current versions of Windows. |
| • | Provide the Windows test team with samples of your device. |
| • | Submit your completed device and associated drivers to WHQL for logo testing. |
