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ZBasic System Library 171 ZX Microcontroller Family where F_CPU is the device’s operating frequency . If the bitRate parameter is omitted or is zero the value of 66 is used by default. Channel 0 Prescaler Selector Value Value Divisor 0 1 1 4 2 16 3 64 For ATxmega-based devices, the I2C bit rate is given by the formula F_CPU / 2 / (5 + rateVal) where rateVal is the low 8 bits of the bitRate parameter. Rearranging this formula gives an equation for the bitRate parameter: bitRate = (F_CPU / 2 / F_I2C) – 5 where F_I2C is the desired I2C clock frequency. For channels 1-4 the bitRate parameter is interpreted as the number of I/O Timer ticks per bit. For I2C operations, the I/O Timer uses the prescaler specified by Register.TimerSpeed1. With the default prescaler of 1, each I/O Timer tick represents approximately 68nS. If the bitRate parameter is omitted or is zero the value of 74 is used by default. Due to processing overhead, the minimum attainable bit time is approximately 4µS. For channel 0, the table below gives the pin numbers used for SDA and SCL. SDA and SCL Pins ZX Models SDA SCL ZX-24, ZX-24a, ZX-24p, ZX-24n, ZX-24r, ZX-24s 11, C.1 12, C.0 ZX-40, ZX-40a, ZX-40p, ZX-40n, ZX-40r, ZX-40s 23, C.1 22, C.0 ZX-44, ZX-44a, ZX-44p, ZX-44n, ZX-44r, ZX-44s 20, C.1 19, C.0 ZX-24e, ZX-24ae, ZX-24ne, ZX-24pe, ZX-24nu, ZX-24pu 11, C.1 12, C.0 ZX-328n, ZX-328l, ZX-32n, ZX-32l 27, C.4 28, C.5 ZX-1281, ZX-1281n 26, D.1 25, D.0 ZX-1280, ZX-1280n 44, D.1 43, D.0 ZX-24x 12, C.0 11, C.1 ZX-32a4 10, C.0 11, C.1 ZX-128a1 15, C.0 16, C.1 ZX-128e, ZX-128ne, ZX-1281e, ZX-1281ne 11, D.1 12, D.0 ZX-328nu 21, C4 22, C5 As noted above, the sdaPin parameter value is ignored when channel 0 is specified. Examples Call OpenI2C(0, 0, 0) ' open the hardware channel at 100KHz Call OpenI2C(2, 19, 20) ' open channel 2 using pins 19, 20 Call OpenI2C(1, C.3, A.1, 74) ' open channel 1 at 200KHz Resource Usage The I2C routines utilize the I/O Timer to regulate the bit timing for channels 1-4. While sending or receiving I2C data, the corresponding timer busy flag will be True indicating that the I/O Timer is in use. On the ZX-24n, ZX-24p, ZX-24r and ZX-24s, the hardware I2C channel cannot be used while Com2 is open since pin 11 is shared by the SDA signal and TxD for Com2.