FPGA芯片：XC7A200T，flash：N25Q256，用Jtag可以正常下载bit文件，但是不能烧写flash。impact的信息如下：
INFO:iMPACT - Current time: 2016/2/18 17:15:59
// *** BATCH CMD : Erase -p 1 -spionly
PROGRESS_START - Starting Operation.
Maximum TCK operating frequency for this device chain: 66000000.
Validating chain...
Boundary-scan chain validated successfully.
1: Device Temperature: Current Reading: -273.00 C
1: VCCINT Supply: Current Reading:   0.000 V
1: VCCAUX Supply: Current Reading:   0.000 V
Unprotect sectors: FALSE
SPI SLAVE TYPE in Cse_Operate.c is 99
key: period_frc, value: 0
key: dclk_has_reset, value: 0
key: period_int, value: 10
Found Slave on Bus Index.
Found Slave on Bus Index.
Slave Int Type is 99, slave Index is 0 , SPI_SLAVE is 99
Slave Type is 99, slave Index is 0
SPI core clock speed value = 0xA801.
key: period_frc, value: 0
key: dclk_has_reset, value: 0
key: period_int, value: 10
Found Slave on Bus Index.
Found Slave on Bus Index.
Slave Int Type is 99, slave Index is 0 , SPI_SLAVE is 99
Slave Type is 99, slave Index is 0
SPI core clock speed value = 0xA801.
PROGRESS_START - Starting Operation.
'1': IDCODE is 'ffffff' (in hex).
'1': ID Check failed.
INFO:Cse - The operation did not complete successfully.
PROGRESS_END - End Operation.
Elapsed time =      0 sec.
25.mcs"
exist.
exist.
exist.
exist.
exist.
it' ...
25.bit"

测量spi flash的电源是正常的，焊接质量还可以，但是片选一直为低电平，clk有固定的38.4MHz时钟。更换flash为M25P64后，现象同上，但是时钟变成了2.7MHz。

求各位大神帮忙解答一下。