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How to connect a device with the memory bus


Article updated at 05 Feb 2018
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This article shows how to connect a device to External Memory Bus.

Please consult the Marvell PXA Datasheet for a complete documentation. In case you are not sure about your design you can contact our support. We do also provide schematic reviews to minimize problems.

Have a look at External Memory Bus article, before reading further.

Devices with dedicated data and address signals

VLIO and SRAM mode are possible, check the datasheet of the device to find out which fits better.

Less than 12 address bits (byte addresses) required

You can use the dedicated data and address signals which are available on all Colibri modules.
Make sure you set up the CPLD on the Colibri PXA3xx modules.

More than 12 address bits (byte addresses) required

Colibri PXA270: You can use 25 dedicated address bits.

Colibri PXA3xx: The CPLD on the Colibri only decodes 12 address bits. If you need more you have different options. You can use GPIOs to emulate additional address bits. In this case you have to set the GPIO before you can access a certain memory area. Or you can decode the multiplexed data/address bus by yourself, instead of using the internal CPLD. In this case you can decode up to 28bits. Use the nLUA and nLLA signals to latch the addresses.

SRAM chips

Sometimes customers need to connect battery buffered SRAM chips. But before you decided to use this solution, make sure you did checked also other solutions, like the internal flash or a flash memory cards.

VLIO and SRAM mode are possible. Right, sometimes it makes sense to connect a SRAM in the VLIO mode. Especially on the Colibri PXA3xx, because normally you need to choose the same memory mode for all external devices.

On the Colibri PXA270 we recommend to use 32bit SRAM (or two 16bit SRAMs) and on the Colibri PXA3xx 16bit SRAM. In case you need a compatible design choose 16bit SRAM for all modules.

8 bit devices

Colibri PXA270: Use 32bit accesses, just ignore the higher 24 data bits.
Connect the address bit 2 of the Colibri to address bit 0 of the device. This maps all device registers to a 32bit aligned address on the Colibri.
We do not recommend using the 16 bit mode.

Colibri PXA3xx: If you want to be compatible with the Colibri PXA270, we recommend doing it the same way then on the Colibri PXA270. Otherwise you can also use a 16bit mode. In this case map the Colibris address bit 1 to the device address bit 0.

Multiplexed address and memory signals (AA/D)

Colibri PXA270: The Colibri PXA270 does not directly support this mode. You need to use external latches. For an example, take a look at the circuit for the CAN chip on the Evaluation Board Schematic.

Colibri PXA3xx: In you would like to be compatible with the Colibri PXA270, see above. If you don’t need to be compatible you can directly use the AA/D interface of the Marvell PXA3xx chip and ignore the dedicated address lines of the Colibri module.