reduce FPGA frequencz to 15MHz to accomodate FPU (#404)

* reduce FPGA frequency to 15MHz to accomodate FPU

* reduced baudrate uart fpga

* update README

README.md

@@ -514,9 +514,20 @@ to load the binaries with the HS2 cable over JTAG,
 or follow the [ExecuteFromFlash](./ExecuteFromFlash.md)
 guide if you have a FLASH attached to the FPGA.
 
-
 Do not forget that the `pynq-z2` board requires you to have the ethernet cable attached to the board while running.
 
+For example, if you want to run your application using flash_exec, do as follow:
+
+compile your application, e.g. `make app PROJECT=example_matfadd TARGET=pynq-z2 ARCH=rv32imfc LINKER=flash_exec`
+
+and then follow the [ExecuteFromFlash](./ExecuteFromFlash.md) to program the flash and set the boot buttons on the FPGA correctly.
+
+To look at the output of your printf, run in another terminal:
+
+`picocom -b 9600 -r -l --imap lfcrlf /dev/ttyUSB2`
+
+Please be sure to use the right `ttyUSB` number (you can discover it with `dmesg --time-format iso | grep FTDI` for example).
+
 
 ### Linux-FEMU (Linux Fpga EMUlation)
 

docs/source/How_to/ExecuteFromFlash.md

@@ -19,6 +19,8 @@ These three modes are mainly controlled by the two inputs pins
 | 1			       | 1				              | SPI Flash Execution  |
 | 1			       | 0				              | SPI Flash Loading    |
 
+
+On the FPGA, such inputs are mapped to two switch buttons.
 Below, a description of the three modes is provided.
 
 ### JTAG Boot Procedure

hw/fpga/scripts/xilinx_generate_clk_wizard.tcl

@@ -5,7 +5,7 @@
 
 set design_name      xilinx_clk_wizard
 set in_clk_freq_MHz  125
-set out_clk_freq_MHz 20
+set out_clk_freq_MHz 15
 
 # Select board
 set_property -name "board_part_repo_paths" -value "[file normalize "../../../hw/fpga/board_files/"]" -objects [current_project]
@@ -25,7 +25,7 @@ set_property -dict [ list \
  CONFIG.CLKIN1_JITTER_PS {80.0} \
  CONFIG.CLKOUT1_JITTER {172.798} \
  CONFIG.CLKOUT1_PHASE_ERROR {96.948} \
- CONFIG.CLKOUT1_REQUESTED_OUT_FREQ {20} \
+ CONFIG.CLKOUT1_REQUESTED_OUT_FREQ {15} \
  CONFIG.MMCM_CLKFBOUT_MULT_F {8.000} \
  CONFIG.MMCM_CLKIN1_PERIOD {8.000} \
  CONFIG.MMCM_CLKOUT0_DIVIDE_F {50.000} \

sw/device/target/pynq-z2/x-heep.h

@@ -12,8 +12,8 @@ extern "C" {
 #endif  // __cplusplus
 
 
-#define REFERENCE_CLOCK_Hz 20*1000*1000
-#define UART_BAUDRATE 115200
+#define REFERENCE_CLOCK_Hz 15*1000*1000
+#define UART_BAUDRATE 9600
 #define TARGET_PYNQ_Z2 1
 
 /**