TS-7840 FPGA PWM
The TS-7840 includes a PWM core that supports 10-bit duty/period, a 50 MHz input clock, and 12 values of input clock shift.
Linux supports this API through the /sys/ interface using file I/O. First export the pwm channel to enable it:
# Export PWM channel 0
echo 0 >/sys/class/pwm/pwmchip60/export
| File | Description |
|---|---|
| /sys/class/pwm/pwmchip60/pwm0/period | Period in nanoseconds. Must be bigger than the duty cycle or writes will fail. Can only change when the pwm is disabled. |
| /sys/class/pwm/pwmchip60/pwm0/duty_cycle | Duty cycle in nanoseconds. Can change at any time, must be less than period. |
| /sys/class/pwm/pwmchip60/pwm0/enable | When 1, pwm is outputting. When 0, outputs idle state of the PWM. |
| /sys/class/pwm/pwmchip60/pwm0/polarity | When "normal", idle high and duty cycle low. When "inversed", idle low and duty cycle high. |
For example, for a 50hz signal with 25% duty cycle:
# Set Period to 20ms
echo 20000000 > /sys/class/pwm/pwmchip60/pwm0/period
# Set duty cycle to 5ms
echo 5000000 > /sys/class/pwm/pwmchip60/pwm0/duty_cycle
# Enable PWM and output 50hz signal
echo 1 > /sys/class/pwm/pwmchip60/pwm0/enable
# Duty cycle can be changed while it is enabled
echo 1000000 > /sys/class/pwm/pwmchip60/pwm0/duty_cycle
The Linux PWM API will attempt to arrive at the exact period at the cost of the duty cycle resolution. For the most possible duty cycle resolution use one of the max period ns values from the table below.
| Shift | PWM Input Frequency (hz) | Max Period (ns) | Max Period (hz) |
|---|---|---|---|
| 0 | 50000000 | 20460 | 48876 |
| 1 | 25000000 | 40920 | 24438 |
| 2 | 12500000 | 81840 | 12219 |
| 3 | 6250000 | 163680 | 6109 |
| 4 | 3125000 | 327360 | 3055 |
| 5 | 1562500 | 654720 | 1527 |
| 6 | 781250 | 1309440 | 1210 |
| 7 | 390625 | 2618880 | 382 |
| 8 | 195312 | 5237773 | 191 |
| 9 | 97656 | 10475547 | 95 |
| 10 | 48828 | 20951093 | 48 |
| 11 | 24414 | 41902187 | 24 |
If period is set to one of these values, the full 10 bits of duty cycle is available. Past that, the Linux API will use the closest available value. Debug output can be enabled with:
echo "file pwm-ts.c +p" > /sys/kernel/debug/dynamic_debug/control
If this is enabled, the kernel can output additional information after setting a frequency:
echo 0 > /sys/class/pwm/pwmchip60/export
# 10ms period:
echo 10000000 > /sys/class/pwm/pwmchip60/pwm0/period
# 5ms duty cycle:
echo 5000000 > /sys/class/pwm/pwmchip60/pwm0/duty_cycle
echo 1 > /sys/class/pwm/pwmchip60/pwm0/enable
dmesg | tail
This will output:
[ 1086.170695] ts-pwm e0000600.mikro_pwm: cycle=2049180 shift=10 cnt=976 [ 1086.170709] ts-pwm e0000600.mikro_pwm: shift=10 cnt=976 duty_cnt=49 [ 75.758146] ts-pwm 500001a8.mikro_pwm: cycle=1293661 shift=10 cnt=773 [ 75.758184] ts-pwm 500001a8.mikro_pwm: shift=10 cnt=773 duty_cnt=387
The last value in cnt indicates how much resolution is available for the duty cycle at this given period. In the best case there are 10 bits (0-2047) to specify duty cycle, but this above example is 0-976 to arrive at this particular period. You can determine the duty cycle increments with period / cnt.
1000000000 / 1023 = 977517
The duty cycle can then be configured in increments of 977517ns. Smaller values will round to the closest value.
This PWM will allow a max speed of 50MHz / 3 = 16.6MHz, but this will sacrifice all of the available duty cycle except an on/50%/off. The slowest speed is highest divisor at 24hz.
While the Linux driver is recommended for most users, the PWM core is located on the FPGA on PCIe BAR0 + 0x600.
| Offset | Bits | Description |
|---|---|---|
| 0x0 | 15:2 | Reserved |
| 1 | Inversed (0 = idle high, duty cycle low), (1 = idle low, duty cycle high) | |
| 0 | Enabled | |
| 0x2 | 15:10 | Reserved |
| 9:0 | Period | |
| 0x4 | 15:10 | Reserved |
| 9:0 | Duty Cycle | |
| 0x6 | 15:4 | Reserved |
| 3:0 | shift (Clock frequency = 50000000 / (1 >> shift)) |