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-rw-r--r--stm-fmc.c46
1 files changed, 28 insertions, 18 deletions
diff --git a/stm-fmc.c b/stm-fmc.c
index 58df6fe..1019531 100644
--- a/stm-fmc.c
+++ b/stm-fmc.c
@@ -153,7 +153,18 @@ void fmc_init(void)
fmc_timing.BusTurnAroundDuration = 0;
// use smallest allowed divisor for best performance
- fmc_timing.CLKDivision = 2;
+ //
+ // FMC_CLK = HCLK / CLKDivision, HCLK is 180 MHz
+ //
+ // Allowed values for CLKDivision are integers >= 2.
+ //
+ // Division == 2: FMC_CLK = 180 / 2 = 90 MHz (highest allowed frequency)
+ // Division == 3: FMC_CLK = 180 / 3 = 60 MHz (one step below)
+ // ...
+ //
+
+// fmc_timing.CLKDivision = 2; // 90 MHz
+ fmc_timing.CLKDivision = 3; // 60 MHz
// use min suitable for fastest transfer
fmc_timing.DataLatency = 4;
@@ -164,22 +175,21 @@ 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.
+ // 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;
- }
+ // 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;
+ }
}