K20 (Kinetis) SRAM interfacing with Flexbus question

[sherry_zahid]

Hi,

I had a few questions about connecting external SRAM with the K20 (specifically part# MK20FN1M0VMD12) flexbus. The SRAM in question is a cypress 256KB SRAM (part # CY7C1011DV33). Q1) Am I going to need pull resistors on the address/data pins of the flexbus? Are pull resistors recommended? (I could not find this information in the data sheets or reference manual).

Q2)What state will these pins be upon reset?

Q3) If I am not using some address pins e.g. A3 and B2, will it be okay to simply leave these pins not connected and floating?

It would really help me out if I got some answers and reasoning for these questions. Many thanks in advance.

Kind regards,

Sherry

[xiangjun_rong · Accepted solution]

Hi, Sherry,

For the MK20FN1M0VMD12 chip, the address/data bus are FB_AD[31:0], the dedicated address bus pins are FB_A[29:16], if you use low address bus FB_AD[15:0], you have to use latch, because FB_AD[15:0] are multiplexed with data bus.

I do not think the pull-up resistor is necessary for data/address bus. Adding pull-up resistor can enhance the driving capability of the pin, if you do not connect multiple memory chips, the driving capability of FlexBus is enough.

The default state of the FlexBus pins is case by case, pls refer to the section 10.3.1 K20 Signal Multiplexing and Pin Assignments, the "Default state" is the pin's state after reset. For example, for FB_A29/PTA24, the default state after reset is CMP_IN4 function.

For the unused pin,if it is output, just have it float, if the pin is input pin, connecting a 10K ohm pull-up or pull-down resistor, it is okay.

BR

Xiangjun Rong

[sherry_zahid]

Hi Xiangjun

Thank you for your response and your time! It was quite helpful. Have a great day

[xiangjun_rong]

Hi, Sherry Zahid,

Of course, the MK20FN1M0VMD12 has FlexBus module and corresponding FlexBus pin which enable you use External SRAM to extend the internal SRAM. I think the application note an4393 is helpful to understand the FlexBus interface.

Pls download an4393 from the website;

http://cache.freescale.com/files/microcontrollers/doc/app_note/AN4393.pdf?fsrch=1&sr=1&pageNum=1

For the address bus, if the address bus pins is multiplexed with data bus pins, the latch for example 74LS573 is needed to latch the address in address cycle of FlexBus. As you know that the  CY7C1011DV33 is a 16 bits data bus SRAM, you have to connect the FB_A1 of FlexBus to A0 of SRAM, FB_A2 of FlexBus to A1 of SRAM,...,connect the data bus one by one, connect the FB_CSx of FlexBus to CE of SRAM, FB_OE to OE pin of SRAM, FB_RW to WE of SRAM. regarding the byte selection, you can connect FB_BE_7_0 to BLE of SRAM, connect FB_BE_15_8 to BHE, it is okay.

Hope it can help you.

BR

xiangjun Rong

[sherry_zahid]

Hi Xiangjun,

Thank you for your response. I am using the pins FB_AD1 to FB_AD15, FB_A16 and FB_A17 (17 pins) to connect to the SRAM address bus (A0-A16) and I am using FB_AD16 to FB_AD31 (16 pins) to connect to the SRAM databus. Do I really need the 74LS537 latch for proper addressing? What can happen if I do not use the latch? Also, I still need to know the answers from the question in my first post as follows:

Am I going to need pull resistors on the address/data pins of the flexbus that I am using?

What state will these pins be upon reset?

I am not using some flexbus pins (e.g. FB_AD0 and FB_A18) will it be okay to simply leave these pins not connected and floating (no pull up resistor required)?

Thanks in advance, kind regards,

Sherry Zahid

[CCandido]

Hi Sherry Zahid,

see it 256 Ksram

MK64FN1M0VMD12