hi all. can 112pin package 9s12xdt256 use external SRAM? datasheet said that 112pin package without external bus interface.thanks!

[jeffzxg]

hi all. can 112pin package 9s12xdt256 use external SRAM? datasheet said that 112pin package without external bus interface.thanks!

[kef2]

True. DATA bus signals have no corresponding pins on 112 pin package. But 144 pin package is as big as 112 pin package...

[jeffzxg]

THANKS karpicz! another question,  in datasheet 112pin package has 23 bit address bus(PORTA PORTB PORTK) and 8bit data bus (PORTC  high 8bit ) , if i use one chip 256k*8 SRAM , does it means that 9s12xdt256 only see 128K byte with ODD address of 256k sram? 

[RadekS]

In case of S12XD, the data bus is always 16 bit. If you want to use 8 bit access then you can disable higher data byte (EBICTL0_HDBE=0) but it does not change addressing mode and data size. It means that higher byte is always written to the port C and lower byte to port D. Because of this, even we want to address 256kB 8bit RAM we have to use 512kB MCU’s external bus address space.

I would like to recommend our application note AN2708 An Introduction to the External Bus Interface on the HCS12X:

http://www.freescale.com/files/microcontrollers/doc/app_note/AN2708.pdf

In attachment you can find also draft of Addendum to AN2708 with practical examples/recommendations.

Note (out of topic): When we use external bus, we cannot secure MCU.

[kef2]

It seems you got it wrong. You don't have external memory bus interface on 112 pin devices. DATA bus is not bonded out of chip, no PORTC(data hi) and PORTD (data lo).

If you meant using 144pin device with single 8bit SRAM, then no, 8bit mode doesn't mean you can't use 1/2 of available RAM. 8bit mode means

• you have data only on odd or even addresses. Like byte-garbage-byte-garbage-...
• you can't execute any code from single external 8bit device
• you have potentially 2 times lower data troughput (you can't store 16bits word in single bus cycle to word aligned address)

For me 8bit mode doesn't make a lot of sense. For 256k you need to have about 18 address pins + about 4 control pins. So 8bit vs 16bit is ~30 vs ~38pins. Is it really big difference?