CODEWITHSUNDEEP
vhdldatabus
JakkeFire
JakkeFire

Reputation: 99

vhdl 32bit counter on 16bit bus

what is the best practice to access a changing 32bit register (like a counter) through a 16bit databus ? I suppose i have to 'freeze' or copy the 32bit value on a read of the LSB until the MSB is also read and vise versa on a write to avoid data corruption if the LSB overflows to the MSB between the 2 accesses. Is there a standard approach to this ?

Upvotes: 1

Views: 140

Answers (2)

Yann Vernier
Yann Vernier

Reputation: 15887

As suggested in both the question and Morten's answer, a second register to hold the value at the time of the read of the first half is a common method. In some MCUs this register is common to multiple devices, meaning you need to either disable interrupts across the two accesses or ensure ISRs don't touch the extra register. Writes are similarly handled, frequently in the opposite order (write second word temporary storage, then write first word on device thus triggering the device to read the second word simultaneously).

There have also been cases where you just can't access the register atomically. In such cases, you might need to implement additional logic to figure out the true value. An example of such an algorithm assuming three reads take much less than 1<<15 counter ticks might be:

earlyMSB = highreg;
midLSB = lowreg;
lateMSB = highreg;
fullword = ((midLSB<0x8000 ? lateMSB : earlyMSB)<<16) | midLSB;

Other variants might use an overflow flag to signal the more significant word needs an increment (frequently used to implement that part of the counter in software).

Upvotes: 1

Morten Zilmer
Morten Zilmer

Reputation: 15924

There is no standard way, but an often used approach is to make read one address return the first 16 bits, while the remaining 16 bits are captured at the same time, and read later at another address.

Upvotes: 1

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