The RP2350 chip in the Raspberry Pi Pico 2, and other RP2350 boards, supports an external interface to PSRAM. When a PSRAM chip is attached to the processor (please note that there is none on the Pico 2 board, but iLabs and SparkFun boards, among others, do have it), up to 16 megabytes of additional memory can be used by the chip.
While this external RAM is slower than the built-in SRAM, it is still able to be used in any place where normal RAM would be used (other than for memory-mapped functions and statically initialized variables).
When present, PSRAM can be used in two ways: for specific instantiated variables, or through a malloc-like access method. Both can be used in any single application.
Similar to PROGMEM in the original Arduino AVR devices, the variable decorator PSRAM can be added to map a variable into PSRAM. Simply add PSRAM to an array and it will be mapped into PSRAM:
... float weights[4000] PSRAM; // Place an array of 4000 floats in PSRAM char samplefile[1'000'000] PSRAM; // Allocate 1M for WAV samples in PSRAM ...
These variables can be used just like normal ones, no special handling is required. For example:
char buff[4 *1024 * 1024]; // 4MB array
void initBuff() {
bzero(buff, sizeof(buff));
for (int i = 0; i < 4 *1024 * 1024; i += 4096) {
buff[i] = rand();
}
}
The only restriction is that these variables may not be initialized statically. The following example will NOT work:
char buff[] = "This is illegal and will not function";Using PSRAM for dynamic allocations
PSRAM can also be used as a heap for dynamic allocations using pmalloc and pcalloc. These calls function exactly like normal malloc and calloc except they allocate space from PSRAM.
Simply replace a malloc or calloc with pmalloc or pcalloc to use the PSRAM heap. Other calls, such as free and realloc “just work” and do not need to be modified (they check where the passed-in pointer resides and do the right thing automatically).
For example, to create and modify large buffer in PSRAM:
void ex() {
int *buff;
// Ignoring OOM error conditions in example for brevity
buff = (int *)pmalloc(10000 * sizeof(*buff));
// Something happened and we need more space, so...
buff = (int *)realloc(buff, 20000 * sizeof(*buff)); // buff now has 20K elements
for (int i = 0; i < 20000; i++) {
buff[i] = i;
}
// Do some work, now we're done
free(buff);
}
C++ objects can be allocated in PSRAM using “placement new” constructors. Note that this will only place immediate object data in PSRAM: if the object creates any other objects via new those objects will be placed in normal RAM unless the object also uses placement new constructors.
The rp2040 helper object has the following calls to return the state of the PSRAM heap with the following calls, similar to the normal RAM heap:
Return the total size of the attached PSRAM chip. This is the RAW space and does not take into account any allocations for static variables or dynamic allocations. (i.e. it will return 1, 2, 4, 8, or 16MV depending on the chip).
int rp2040.getTotalPSRAMHeap()Returns the total PSRAM heap (free and used) available or used for pmalloc allocations.
Returns the total used bytes (including any overhead) of the PSRAM heap.
int getFreePSRAMHeap()Returns the total free bytes in the PSRAM heap. (Note that this may include multiple non-contiguous chunks, so this is not always the maximum block size that can be allocated.)
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