本人初学fpga，用quartus II 64，vhdl写的代码，不知道为什么一个简单的代码都编译不通过。
源代码：
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
ENTITY buzzer IS
PORT (
clk : IN std_logic;
dout : OUT std_logic);
END buzzer;
ARCHITECTURE arch OF buzzer IS
SIGNAL num: std_logic_vector(31 downto 0);
SIGNAL vet: std_logic := '0';
BEGIN
PROCESS(clk)
BEGIN
IF(clk'event and clk ='1') THEN
num <= num + 1;
IF(num = 48000) THEN
num <= "00000000000000000000000000000000";
vet <= vet XOR '1';
END IF;
END IF;
END PROCESS;
dout <= vet;
END arch;
不知道下列错误原因，求大佬给个解决办法，
错误：
Internal Error: Sub-system: FSAC, File: /quartus/fitter/fsac/fsac_lvds_mgr.cpp, Line: 5347
m_util->m_max_non_serdes_ppds_datarate != DTM_ILLEGAL_DELAY
Stack Trace:
0x8b10d: FSAC_LVDS_MGR::init_constant_information + 0xe7d (FITTER_FSAC)
0x8e029: FSAC_LVDS_MGR::FSAC_LVDS_MGR + 0x419 (FITTER_FSAC)
0x8e0ca: FSAC_LVDS_MGR::create_manager + 0x4a (FITTER_FSAC)
0x5c88: fcuda_execute + 0x14f8 (fitter_fcuda)
0x8d89: fcuda_execute + 0x45f9 (fitter_fcuda)
0x4509f: FITCC_EXPERT::fitter_preparation + 0x23f (FITTER_FITCC)
0x46ed7: FITCC_EXPERT::invoke_fitter + 0x427 (FITTER_FITCC)
0x497e: fcuda_execute + 0x1ee (fitter_fcuda)
0xb55d: fmain_start + 0x7cd (FITTER_FMAIN)
0x1264b: qexe_get_command_line + 0x1c5b (comp_qexe)
0x1588d: qexe_process_cmdline_arguments + 0x5ad (comp_qexe)
0x159a1: qexe_standard_main + 0xa1 (comp_qexe)
0x1a48: MSG_INITIALIZER::~MSG_INITIALIZER + 0x118 (CCL_MSG)
0x19ec: MSG_INITIALIZER::~MSG_INITIALIZER + 0xbc (CCL_MSG)
0x53e4: MEM_INITIALIZER::~MEM_INITIALIZER + 0x244 (ccl_mem)
0x3379f: msg_exe_main + 0x8f (CCL_MSG)
0x12773: BaseThreadInitThunk + 0x13 (KERNEL32)
0x70d50: RtlUserThreadStart + 0x20 (ntdll)
End-trace
Quartus II 64-Bit Version 11.1 Build 216 11/23/2011 SJ Full Version
Service Pack Installed: 1
