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4 communicate with other SPI devices. Although most SPI devices are tolerant of the ZX device using the SPI bus to fetch instructions from your program, a few are not. Generally speaking, if you can send and receive all of the data that an SPI device requires using a single call to SPICmd(), then that SPI device is usable with the ZX models that utilize an external EEPROM. The table below indicates which devices use an external EEPROM for user programs and, if so, the I/O pin used for the chip select. SPI Channel and EEPROM Usage ZX Model Uses SPI EEPROM SPI CS Pin ZX-24, ZX-24a, ZX-24p Yes B.4 ZX-40, ZX-40a, ZX-40p Yes 5, B.4 ZX-44, ZX-44a, ZX-44p Yes 44, B.4 ZX-24e, ZX-24ae, ZX-24pe Yes 24, B.4 ZX-24n, ZX-40n, ZX-44n No B.4 ZX-24ne No 24, B.4 ZX-1281, ZX-1281n No 10, B.0 ZX-1280, ZX-1280n No 19, B.0 ZX-128e, ZX-128ne, ZX-1281e, ZX-1281ne No 28, B.0 It is important to note that even for the devices that do not use the external SPI EEPROM, the SPI CS pin cannot be used as a general input if the SPI bus is used in your application. This restriction is an artifact of the design of the CPU’s SPI controller. The SPI CS pin can, however, be used as a general purpose output. Analog-to-Digital Converters Most ZX devices support up to 8 analog inputs. These inputs may be fed to the internal analog-to-digital converter (ADC) or they may be used to perform analog level comparisons. The I/O port containing the analog inputs varies by ZX device as indicated in the table below. The System Library routines GetADC() and ADCtoCom1() use the ADC. The analog comparator is used by WaitForInterrupt() when configured to await an analog comparator event. Analog Input Ports by CPU Type Underlying CPU Type Analog Port 1 Analog Port 2 mega32, mega644, mega644P Port A - mega128, mega1281 Port F - mega1280 Port F Port K Interrupts Some of the System Library routines disable interrupts in order to achieve the precise timing that is required. Having interrupts disabled for long periods of time can interfere with the operation of other parts of the system that use interrupts like task management, serial I/O and the real time clock. In most cases, the System Library routines have been implemented to keep track of real time clock interrupts that should have occurred during the time interrupts are disabled and then the RTC is updated at the end of the operation. This strategy avoids the problem of the RTC losing time. Unfortunately, there is no way to similarly protect the serial I/O process. You can reduce the impact of having interrupts disabled with respect to serial output by ensuring that all serial output queues are empty before calling a System Library routine that disables interrupts. This is not as critical for a hardware- based serial channel (e.g. Com1) as it is for the software-based serial channels Com3 to Com6. There is no way, however, to work around the problem of serial input data arriving while interrupts are disabled. The hardware-based serial channels will store one received character and hold it while interrupts are disabled but if a second character arrives while interrupts are disabled it will be lost. Channels 3-6 rely on interrupts for every bit received so the situation is much more problematic. In this case, having interrupts disabled for longer than approximately one-third of the bit time will likely cause garbled input if a character’s transmit time overlaps the period when interrupts are disabled. For characters being