/** * Copyright (c) 2020 Raspberry Pi (Trading) Ltd. * * SPDX-License-Identifier: BSD-3-Clause */ #include <stdio.h> #include "pico/stdlib.h" #include "hardware/structs/watchdog.h" // This app shows the effect of byte and halfword writes on IO registers. Most // IO registers sample the entire 32 bit write data bus on any write; the // transfer size and the 2 LSBs of the address are *ignored*. // // This can have unintuitive results, especially given the way busmasters // replicate narrow write data across the entire 32-bit write data bus. // However, this behaviour can be quite useful if you are aware of it! int main() { stdio_init_all(); // We'll use WATCHDOG_SCRATCH0 as a convenient 32 bit read/write register // that we can assign arbitrary values to io_rw_32 *scratch32 = &watchdog_hw->scratch[0]; // Alias the scratch register as two halfwords at offsets +0x0 and +0x2 volatile uint16_t *scratch16 = (volatile uint16_t *) scratch32; // Alias the scratch register as four bytes at offsets +0x0, +0x1, +0x2, +0x3: volatile uint8_t *scratch8 = (volatile uint8_t *) scratch32; // Show that we can read/write the scratch register as normal: printf("Writing 32 bit value\n"); *scratch32 = 0xdeadbeef; printf("Should be 0xdeadbeef: 0x%08x\n", *scratch32); // We can do narrow reads just fine -- IO registers treat this as a 32 bit // read, and the processor/DMA will pick out the correct byte lanes based // on transfer size and address LSBs printf("\nReading back 1 byte at a time\n"); // Little-endian! printf("Should be ef be ad de: %02x ", scratch8[0]); printf("%02x ", scratch8[1]); printf("%02x ", scratch8[2]); printf("%02x\n", scratch8[3]); // Byte writes are replicated four times across the 32-bit bus, and IO // registers usually sample the entire write bus. printf("\nWriting 8 bit value 0xa5 at offset 0\n"); scratch8[0] = 0xa5; // Read back the whole scratch register in one go printf("Should be 0xa5a5a5a5: 0x%08x\n", *scratch32); // The IO register ignores the address LSBs [1:0] as well as the transfer // size, so it doesn't matter what byte offset we use printf("\nWriting 8 bit value at offset 1\n"); scratch8[1] = 0x3c; printf("Should be 0x3c3c3c3c: 0x%08x\n", *scratch32); // Halfword writes are also replicated across the write data bus printf("\nWriting 16 bit value at offset 0\n"); scratch16[0] = 0xf00d; printf("Should be 0xf00df00d: 0x%08x\n", *scratch32); }

This firmware image was imported from the pico-examples repository. 


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