Updated FMC initialization code to match changes in FMC arbiter.

1 files changed, 25 insertions, 22 deletions

diff --git a/stm-fmc.c b/stm-fmc.c
index 58df6fe..62b6433 100644
--- a/stm-fmc.c
+++ b/stm-fmc.c
@@ -62,7 +62,7 @@ void fmc_init(void)
      */
     GPIO_InitStruct.Pin = GPIO_PIN_6;
     GPIO_InitStruct.Mode = GPIO_MODE_INPUT;
-    GPIO_InitStruct.Pull = GPIO_PULLUP;
+    GPIO_InitStruct.Pull = GPIO_PULLDOWN;
     HAL_GPIO_Init(GPIOD, &GPIO_InitStruct);
 
     fmc_af_gpio(GPIOE, GPIO_PIN_2
@@ -152,10 +152,10 @@ void fmc_init(void)
     // not needed, since nwait will be polled manually
     fmc_timing.BusTurnAroundDuration = 0;
 
-    // use smallest allowed divisor for best performance
-    fmc_timing.CLKDivision = 2;
+    // use 45 MHz to match what FMC arbiter in the FPGA expects
+    fmc_timing.CLKDivision = 4;
 
-    // use min suitable for fastest transfer
+    // use 4 to match what FMC arbiter in the FPGA expects
     fmc_timing.DataLatency = 4;
 
     // don't care in sync mode
@@ -164,22 +164,25 @@ void fmc_init(void)
     // initialize fmc
     HAL_SRAM_Init(&_fmc_fpga_inst, &fmc_timing, NULL);
 		
-		// STM32 only enables FMC clock right before the very first read/write
-		// access. FPGA takes certain time (<= 100 us) to lock its PLL to this frequency,
-		// so a certain number of initial FMC transactions may be missed. One read transaction
-		// takes ~0.1 us (9 ticks @ 90 MHz), so doing 1000 dummy reads will make sure, that FPGA
-		// has already locked its PLL and is ready. Another way around is to repeatedly read
-		// some register that is guaranteed to have known value until reading starts returning
-		// correct data.
-		
-		// to prevent compiler from optimizing this away, we pretent we're calculating sum
-		int cyc;
-		uint32_t sum;
-		volatile uint32_t part;
-		
-		for (cyc=0; cyc<1000; cyc++)
-		{
-			part = *(__IO uint32_t *)FMC_FPGA_BASE_ADDR;
-			sum += part;
-		}
+	// STM32 only enables FMC clock right before the very first read/write
+	// access. FPGA takes certain time (<= 100 us) to lock its PLL to this frequency,
+	// so a certain number of initial FMC transactions may be missed. One read transaction
+	// takes ~0.22 us (10 ticks @ 45 MHz), so doing ~500 dummy reads will make sure, that FPGA
+	// has already locked its PLL and is ready. Another way around is to repeatedly read
+	// some register that is guaranteed to have known value until reading starts returning
+	// correct data.
+
+	// to prevent compiler from optimizing this away, we pretent we're calculating sum
+	int cyc;
+	uint32_t sum = 0;
+	volatile uint32_t part;
+
+	for (cyc=0; cyc<500; cyc++)
+	{
+		part = *(__IO uint32_t *)FMC_FPGA_BASE_ADDR;
+		sum += part;
+	}
+
+	// dummy write to read-only address to not let the compiler remove the above loop
+	*(__IO uint32_t *)FMC_FPGA_BASE_ADDR = sum;
 }