Frequency Counter

Frequency Counter
Status	Stable
menuconfig	Applications->Frequency Counter
Pinning	yes
Ecmd	yes
Depends on	ECMD (optional)
Requires	-
Code	https://github.com/ethersex/ethersex/tree/master/services/freqcount

Measures the frequency and duty cycle of a signal

Pinning

• the signal must always be fed to the ICP1-pin. Look up in the datasheet which physical pin this is on the used controller.
• you need to define the pinning in pinning/hardware/<your board>.m4:

 pin(FREQCOUNT_PIN, PB0, INPUT)

• if you want to measure the frequency of multiple signals, you can multiplex them with one or two 74HC251
• define the pinning for the control lines you need like this:

 pin(FREQCOUNT_CHANNEL_MULTIPLEX_BIT1, PB1, OUTPUT)
 pin(FREQCOUNT_CHANNEL_MULTIPLEX_BIT2, PB2, OUTPUT)
 pin(FREQCOUNT_CHANNEL_MULTIPLEX_BIT2, PB3, OUTPUT)
 pin(FREQCOUNT_CHANNEL_MULTIPLEX_CS_A, PB4, OUTPUT)
 pin(FREQCOUNT_CHANNEL_MULTIPLEX_CS_B, PB5, OUTPUT)

Frequency

• CPU ticks from rising edge to rising edge are measured
• minimum frequency is the smaller value of 1 Hz and CPU frequency divided by 16777216, in practice 2 Hz
• if the frequency is below the minimum, the output is 0
• maximum frequency is about 50 cpu ticks, about 400 KHz at 20 MHz CPU
• if the frequency is above the maximum you get bogus results
• when the irq load of the cpu increases (e.g. by network traffic or UART), the maximum frequency decreases

Duty Cycle

• you can also measure the duty cycle of a PWM signal
• duty cycle is (time from rising edge to falling edge) / time of whole cycle from rising to rising
• frequency and duty cycle are measured one after another
• when the frequency changes significantly the measured duty cycle will be wrong
• duty cycle is stored as 8 bit value

Durchschnittsbildung

• Durch Interrupt-Verzögerungen etc. kann es zu Fehlmessungen kommen
• Bei höheren Frequenzen sinkt die maximal mögliche Auflösung
• Um das zu kompensieren wird immer der Durchschnitt von n Messungen verwendet, n ist im Konfigurationsmenü einstellbar
• Zusätzlich wird immer der höchste und der niedrigste Wert einer dieser Durchschnittsreihen verworfen (es werden n+2 Samples genommen)

ECMD-Befehle

Hinweis: Channel ist momentan immer 0

Kommando	Funktion
fc freq Channel	Gibt die Frequenz in Hz zurück (Achtung: 32 Bit).
fc ticks Channel	Gibt die Frequenz in CPU Ticks zurück (Achtung: 32 Bit).
fc duty Channel	Gibt den Duty Cycle als 8 Bit Wert zurück (0-255 dezimal).
fc %duty Channel	Gibt den Duty Cycle in Prozent zurück.